I can see maybe when its raining but why do some people wear bandana's over there face when they are riding especially on hot days? An old fashion trend?
I do it if the bugs are really bad out that day. I can only take so many to the face before I pull over and put my bandanna on. It helps take away some of the sting from impact.
I wear one in the springtime to filter out some of the pollen. When you have allergies, you have to do all you can. Some go to full face helmets but I don't like those.