I do this for various reasons - keeping my hair from sticking out on the sides of the helmet (which looks stupid with my hair), being able to put the helmet on easier when my hair isn't in the way, and looking not as terrible when I take the helmet of. Also I wear warmer Buffs when it's colder to keep my ears from freezing. And the Bandanas/Hankies can be washed easily and keep my sweat from running into the helmet as much.