On the mountain bike, buff under the helmet.
On the road/cross bike, little gay roadie cap, back to front normally, right way round if rainy.
Both make the helmet a bit more comfy – I have very little hair – no 1 all over. In the summer, they prevent sunburn. In the winter they keep the wind off. When it's rainy, the peak keeps the rain out of the eyes when glasses would probably steam up.