History, protection, style, attitude
Welcome to the modern leather jacket.
Humans are a squishy creature. We shiver in the cold. We scrape and bleed on the rocks. We break when we fall too hard. Most of the creatures we think are tasty were, originally, big enough to think we were tasty too - or at least, break us if they got pissed off. Which, you might expect, they would tend to get if we rolled up and fronted like we were going to eat them.
Somehow, though, we ended up owning the planet, if owning implies they who have the power to fuck it up.
We're adaptable. We have brains. How many brains is debatable; but enough of them that at some point, some human or proto-human looked at the beast which he or she had killed to eat, remembered how hard it had been to pull that off, and thought "Hey, if their skin was that hard to get through, maybe I can steal it...they're not gonna need it anymore..."
Humans have been wrapping themselves in animal skins to keep warm and safe since long before recorded history. As technology advanced, so did our ability to acquire and make armor; in the long day of human existence, leather has been with us as armor for longer than any other option. First we had to learn to scrape the meat and hair off it so it wouldn't stink so much; then we learned to make it into shapes that fit us better. Or maybe not in that order. We learned to attach those pieces to each other. And so on.
By the twentieth century, leather armor was passé. We had kapok vests, metal rings and plates, quilted fabrics and all manner of other clever materials to keep our fragile selves safe and warm. But for some things, leather was still the cheapest, easiest and most effective. Once we'd domesticated animals, it was easy to get, and we'd learned all manner of ways to soften it (curing, tanning, what-have-you) so it was easy to wear as well as resistant to decay and the weather. Cavalry use brought about the shorter 'jacket' style as opposed to the longer, warmer coat - for use astride a horse, the garment stopped above the waistline to allow the legs to spread comfortably. This form of garment became a natural when people began using vehicles rather than horses, especially narrow ones - motorcyclists began using them nearly as soon as the vehicles were invented.
Mass production was turned to leather jackets with the widespread adoption of aviation - it offered a good blend of protection and warmth, and could be lined easily. The wars of the first half of the twentieth century ensured that the mass-produced leather jacket became an item of familiarity to entire generations all over the world - the bomber jacket of the Allied air forces, for example, or the tanker's jacket. By the time World War Two was over, literally millions of the things had been turned out. One of the first military suppliers of the new design of jacket for the U. S. Army Air Corps, called the bomber jacket, was a company in New York City named Schott - which had started up to make motorcycling jackets. They're still around.
From GI to FU
After the war, the rise of motorcycles was accelerated due to their ease of maintenance and easy availability/affordability. The most famous pop culture icon that folks point to when describing the rise of motorcycling culture is, of course, the film 'The Wild One (1953) starring a sneering Marlon Brando and (even more importantly for our purpose) his leather jacket. Bearing his name on its breast in a nod to its military lineage, Johnny's black leather jacket nevertheless came to epitomize the anti-authoritarian streak in everyone who sympathized with him as he took on the conservative townsfolk and law enforcement in defense of his desire to just go his own way.
The leather jacket, along with the motorcycle, became a huge selling item after that movie, perhaps ironically. Now everyone, even if they were too quiet to say so aloud, could say fuck you, man just by wearing one. Their inclusion in the gang uniform of the T-Birds in Grease would only serve to reinforce the image. Especially as military wear began to migrate away from the simplicity of leather gear and on to synthetic fibers, G-suits, body armor, and camouflage, the leather jacket turned comfortably back to its original market.
The Wheel Turns
Of course, fashion co-opts all. Eventually, being 'out' was 'in' (a lot sooner than anyone expected) and suddenly everyone began turning up in leather jackets despite the scowls of any local Hell's Angel who might see. Movie stars, fashion models, even the average Joe who just wanted a warm jacket for a night around town - and as their popularity grew, the styles softened into a myriad of less-extreme models. Longer backs, softer leather, warmer linings, less metal - all proliferated, and still do. Different types of leather became readily and cheaply available; imported hides, better cured hides, leather optimized for particular colors, even (shudder) dyed pink leather, all were made into jackets and sold for whatever price they could be jacked up to.
The jacket is still around, still expensive if you want a really good one, and now has more optional bells and whistles available than ever. Several companies make 'tech' jackets which contain not only wiring conduits but literally dozens of cleverly hidden pockets to hold the plethora of gadgets the modern geek can't leave the house without. Paranoiacs both professional types and otherwise can buy them with all manner and level of armor protection, from simple rip-stop up to Kevlar and ceramic inserts. New techniques in manufacturing, using laser cutting for example, make custom orders much easier - which allow better fits and quicker turnaround.
One thing hasn't changed, though. If you want one that'll do you good when you take a header off your bike, buy a real one, and take the time to break it in proper. Don't buy 'pre-softened' or 'synthetic pleather' or any of the fun words you see. If you just want protection, there's better stuff available; but for style and safety, it worked fine for cows until they met us. Should do OK for you too.