@roger,
The pool I go to has a separate area for swimming laps. A lot of the men and women wear them.
I'm pretty sure they do it mostly to make themselves more streamlined.
I think also to keep their hair from flopping around, and into their eyes/mouth.
You have to wet your head before you put a swim cap on. Putting it on with dry hair hurts. You should wet your head with fresh water, not the chlorinated pool water. If your hair is saturated with fresh water, it won't as readily soak up the chlorine.
I used to do that, just to protect my hair, but now, I just bring a container of water, drench my hair, then put on some conditioner before I go in the pool. The conditioner creates another barrier.
Yes, I'm careful with my hair. I color it and anyway don't want it to get messed up w/ the chlorine.