An author has many literary devices available when he or she decides to tell a story. One of the oldest of these devices is use of a narrator to tell the tale. This narrator may be a character in the story, an unnamed stranger, or the author himself or herself. Long ago when the world was a bit younger than it is now, authors who chose to place a narrator behind the curtain of their story would address the reader. "Ladies and gentlemen" just didn't work. It wasn't personal. The author needed something that would allow any reader to think they were being directly addressed.
"And now, gentle reader, I begin a tale that must be told..."
The narrator device allows the author an opportunity to address the reader directly, to draw him or her into the story. It also allows the author to "fill in the blanks" in a story landscape that is complex with integral back story, but it is not important to discuss this at this time...
"What terrible rains and wind that storm did bring. If you had stood there amongst them, gentle reader, you would know of what I speak.
In more modern times, the narrator device is still used frequently, but I doubt that in this day and age the art of intimately addressing readers is as popular. "Gentle reader" would sound somewhat spooky or odd in our era. Maybe that is why I am so interested in using it myself.
"And now, gentle reader, I must say goodnight..."