Most Helpful Guy
I wear underwear (specifically briefs) for a few reasons. They provide support for my penis and testicles, keep my penis and testicles from moving around, and provide a layer of protection against chaffing on sensitive skin.
Most Helpful Girl
It does prevent you from soaking through your pants when you get really wet. At least, that's been my experience. Fortunately, that doesn't happen very often when I'm out in public.