If you're talking about a sweatband on the wrist, it's sort of a hygene thing. Notice that most people who use it put it on the part of their forearm that makes contact with their guitar. It's there mostly to stop sweat from dripping all over the guitar, esp for musisicans who sweat a lot or when you play under hot spotlights, etc. Sweat is corrosive and disgusting, especially if it gets into the bridge if you're the type of player who palm mutes a lot or anchor your palm on the bridge a lot. Ever try to clean out a bridge that has been corroded by sweat with cotton buds and sewing machine oil? Eew.