Yet another thing I don't understand

Sometimes, I question whether I'm from this planet or not. So many things that are accepted as standard practices just seem wrong or really, really stupid to me. And it's not that I think I'm smarter than other people and that I have everything figured out while everyone else is clueless. If anything it's probably the opposite and the fact that these things confuse me causes me to wonder what the hell is wrong with me. One example would be the bathroom habits I mentioned recently. Another has to do with motorcycle helmets. First, here's comedian Jerry Seinfeld's take on motorcycle helmets and laws requiring they be worn:

"There are many things you can point to as proof that the human is not smart. But my personal favorite would have to be that we needed to invent the helmet. What was happening, apparently, was that we were involved in a lot of activities that were cracking our heads. We chose not to avoid doing those activities but, instead, to come up with some sort of device to help us enjoy our head-cracking lifestyles. And even that didn't work because not enough people were wearing them so we had to come up with the helmet law. Which is even stupider, the idea behind the helmet law being to preserve a brain whose judgment is so poor, it does not even try to avoid the cracking of the head it's in."

Now, this isn't about the debate over wearing motorcycle helmets. here in Florida, the law requiring motorcyclists to wear helmets was repealed 10 years ago. I don't ride motorcycles so I own neither a cycle nor a helmet. I understand that many motorcyclists have their own personal reasons for wearing or not wearing them and I believe that those reasons are as valid as anyone's personal preferences to do or not do anything else are. I get that some motorcyclists love their freedom and being as close to naked as they can get. I also get that some feel the need to bundle up in as much protective body armor as humanly possible. Again, the rightness or wrongness of it extends as far as the individual's personal preferences. There's a certain romantic appeal for bad-ass biker renegades in dying the way they lived; clad in little more than leather chaps and a bandanna. By the same token, one's natural instinct for self-preservation makes the idea of walking away from a fireball of twisted metal and burning rubber and not having to do much more than buff the scratches out of your custom-painted helmet very appealing. I understand the reasoning behind both of those stances.
Here's what I don't understand...

I see people all the time wearing almost no clothes...but still bothering to strap a helmet on. Why? This makes as much sense to me as a medieval knight going into battle wearing a helmet and a Speedo, which is to say, none. But again, it's entirely possible that this is a shortcoming on my part and that someone reading this right now is thinking, "a series of painful and expensive skin graft procedures after my body is hurled across asphalt at 60 mph like I'm on a water slide made out of cheese graters is something I could miss out on if I were to be killed instantly by not wearing a motorcycle helmet, you dumbass."
Yeah, seriously. What was I thinking?