Not just for sweat absorption, but I always wear a "doo-rag" to protect my bald scalp from sunburn thru the helmet vents.  My mountain biking helmet has a visor, but my road helmet does not.  Therefore, I've started using this type of cap when road biking, because of the lid offering sun protection in the absence of a visor.  Of the 3 or 4 brands I use, this is the most comfortable (and the nicer color doesn't hurt either).