Here's the reason why! I sweat from my head like nobodies business. No matter the type of helmet and the design of pads the sweat will always find it's way into my eyes or all over my sun glasses. Using a cycling cap allows the sweat to run off the visor and it does not get in my eyes or on my sun glasses.
Normally I only have one helmet and if it is a mountain bike helmet I will take the visor off when I am riding on the road. It just does not look right to wear a visor on your helmet while riding on the road. But that's me.
http://www.pacesportswear.com celebrating 30 years of U.S. made cycling apparel. The following caps that I have are made by Pace. The Clif Bar Euro Cycling Cap is a lightweight cotton twill 4 panel dome with traditional flip up visor
5/8 in twill tape sewn inside with elastic back one size fits all.The other two are lightweight cotton twill 3 panel dome with traditional flip up visor KoolFit® elastic sweatband and one size fits all.
I like the 4 panel design better than the 3 panel. Bottom line they are a good product and they work.