I am not going to touch on the biographical details of Mr. Lovecraft, if you're interested at all in his birth, life and death they are well documented both here and on Wikipedia.
As a human being he was overly racist, even by the standards of his day and pretentious. As an author, well, let me remind you that he wrote pulp fiction, a significant amount of which was rejected in his lifetime. He revisited ideas over and over, and many of those were rehashes of other authors. Dagon, the Deep One immortal fish-people of Innsmouth and so on were based on another author's work, "Fishhead".
Let's not get into the fact that he reused some set ideas over and over again: the story as suicide note or warning note to others about the main character's disappearance - using a character going insane or fainting in horror so as to avoid writing What Happens Next, descriptions of things as "gibbous, amorphous, horrifying" etc. etc. and refusing to describe them and so forth, vast underwater cities that send dreams to others (an idea he took from another author, in passing....) and so on.
What he did do however, is create a world, a milieu (since referred to as the Cthulhu Mythos and backdrop for storytelling about cosmic horror that other authors and properties have since used, in an "open source" style arrangement - both during his life and afterwards. Just about everyone has heard of the Necronomicon, or the name Cthulhu, or seen a reference to Arkham or Miskatonic University.
And he is, for the record, important in the history of 20th century horror.
Not only because notaries like Stephen King were lauded for finally breaking the cosmic horror stranglehold that Lovecraft and others had on horror - but because he managed to successfully bridge two distinct generations of horror, and be one of the leading horror writers of his time.
I argue that there are two main ideas in horror - that of some kind of physical threat, which is where gorn, splatterpunk and slasher movies get their strength. The other is contagion. And whereas our worries of contagion are based on the viruses, decay and environmental threats we face in our own iives - his were based on earlier concerns in earlier times.
And keep in mind that there's a difference between a jump scare, fear, and horror. A jump scare can as easily be triggered by a cat jumping through a window as someone showing up in the next frame with an ice pick. Fear is something very immediate - "he's out there!". Horror, on the other hand - is something we're getting increasingly immune to. It's the kind of sensation that someone gets knowing there's some doom ahead. It's the existential dread of religion - knowing that the end of life is an eternal Hell - or the Buddhist idea of reality being a never ending cycle of pain - the pain of birth, growing old, old age, and death. Repeating over and over again. Forever. For most, the closest they will get is the sinking feeling a child gets when told they've crossed a line during a trip to the mall and they know that not now, but at some later moment when the shopping trip is over their pants will be removed and a stinging belt will be applied to their buttocks, beyond their threshold to cope. For hippie raised kids like myself who have never been spanked, it's what you get knowing you will need a painful medical procedure, such as a root canal - one that will happen next Monday. It's not the immediate threat, but the wait, and the dreaded anticipation. One that gets worse as time goes on, which weighs on every moment, one worse than the actual spanking, surgical procedure, etc. in many ways.
His world, his real world - was one where scientific discoveries were sometimes breakthroughs but also sometimes curses. Marie Curie discovered radium and did groundbreaking work in nuclear physics, but died of radiation poisoning that doctors didn't even know how to fix. In fact, even after she died some health food aficionados were drinking radium water which promised to give you "the power of atomic energy!"(TM)* Until one of them keeled over, dead of numerous cancers and people realized to their horror they had unleashed something quite dangerous. As Tom Baker (as Doctor Who) said to Professor Sorenson in Planet of Evil - "You and I are scientists, Professor. We buy our privilege to experiment at the cost of total responsibility." And sometimes that bill carries a LOT of penalties, and interest.
At the same time that Lovecraft's world was understanding more and more about chemistry and physics, new technologies enabled people to travel and intermingle. Explorers could reach further corners of the earth and run into tribes who are now wearing Western clothing and posting on Facebook via cellphones. Artifacts were dug up which revealed doomed civilizations, and Lovecraft's personal interests went beyond the sciences and into archaeology as well.
And a major theme in his work was about our world becoming contaminated by contact - not only with our own experiments, but in contact with other peoples who had made the mistake of poking their noses in to corners where noses should never, ever be poked. Or even worse, breeding with them - in a contract that promised early people short term incredible financial gain in exchange for a long term eradication of the human race.
As someone who grew up during a time of segregation and racial purity, and the monstrous "science" of eugenics - and also with a personal history of sickness and whose parents both died in mental hospitals (and was very probably mentally ill himself) he was terrified of there being something in his own DNA that would "out". It was believed that genes would eventually "out" - after all, what Englishman of a certain age or blue blood New Englander wouldn't have heard at least the concept of "breeding" as applied to people, and the importance of avoiding madness and physical infirmity in the people you bred with.
It's not an idea peculiar to Lovecraft - Bilbo Baggins in the Hobbit was considered to have "gone crazy" and gone off on adventures because of the "Took"ish blood on his mother's side. Bertha Mason in Jane Eyre had the "madness" come out in her "Creole blood". But Lovecraft took it up to 11 in his work, with the idea of the Deep Ones interbreeding with people, and human beings discovering that their skin falling off and their inability to walk anymore was merely a precursor to becoming a scaly fish person worshipping some demonic entity in a dark underwater city, their human personalities eventually eroding as they take on a different, human sacrificing nature. He took his own worry of eventually dying in the same kind of mental sanitarium his parents both died in, as well as his horror of seeing white bloodlines interbred with "darker" people - and stirred up very dark, very primal concerns about "us" and "not us".
The importance of this, historically - is that prior generations of horror writers concentrated on the importance of the purity of the soul. The horror of Faust is Doctor Faustus knowing full well Satan will come for his immortal soul. But apart from breast-filled Italian horror movies of the 1960s, who really cared about medieval morality plays in a modern world where the Bible, taken literally, would become seen as increasingly ridiculous?
Lovecraft took the worry of contaminating one's soul with contact with sins and sinners, and transferred it to the society's fear of where science would poke its nose in next. This isn't a horror peculiar to his generation - I know people who were utterly convinced that some high-energy physics experiment or other would either create a black hole somewhere at Cal Tech or otherwise somehow disturb the electrical house of cards that underpins all of reality and create a literal reality bomb annihilating all space and time. He also took their fear of contamination of the soul by some kind of deal with the devil, and transferred it to body horror in which someone cannot escape a very real and personal doom.
Horror of the body would have been ridiculous in a time where many children didn't see adulthood and the average age at death was somewhere in the 40s. After all, to religious folk thi is a transient state before an eternity in Heaven, who cares about suffering for a year or two? Having accepted that youth is fleeting, life is short and brutish - most cultures would have just scoffed. But once people stopped fearing for their soul and living long enough - they needed something else, some other kind of fear to adhere to. And in Lovecraft's world the Deep Ones never die, except by being killed. Lurking underwater as a fish demon is an eternal sentence, and an endless body horror. There are gigantic demonic forces that don't fit into our Christian/Zoroastrian ideas about good and evil, and have no human advocate in the way Jesus fits into our theological understanding.
Of course, eventually we came to live in a world where even Big Ideas about the soul and existential life were lost, and the only thing you could scare them with was the idea of being cut to pieces by someone weilding a chainsaw. Clive Barker touched on the idea of the universe having energies behind it that didn't play by our rules with Hellraiser - the Cenobites and the Hell Priest specifically - but even then, that was less based on fear of going crazy over decades and was more based on his concerns while dating (as a gay man) that he'd eventually cross paths with someone whose idea of a Tinder date involved blades and pain, and no safeword.
It's why Lovecraft as a person is little remembered, but why more people know about him from references in Metallica songs and shoutouts than his original work. Because they don't really remember him for his own personal achievements, but by tapping hard into the zeitgeist of what scared people. He's also remembered as someone who brought dread and horror back into a genre which had abandoned previous reasons for dread. And as such, he takes an important place in the pantheon of horror.
*These statements were never approved by the FDA and are not meant to diagnose, cure, or treat any disease.