Umm as the name suggest, it's for wrist support. I use it because I'm a wrist player, and sometimes after long badminton sessions my wrist will experience fatigue, and gets weaker. With a wrist band, your wrist receives more support, and fatigues slower. Plus I find my shots more consistant with my wrist band. Just my opinion tho. Not everyone needs one, for instance if you are a arm player.
May, if what you're asking about is made from terry cloth (same materials as towels) then Byro is correct - it is for wiping up sweat (I use them every time I play). Otherwise, look to Chan's response.
That's kind of like saying "which one is best, a strawberry, an apple or an orange?" It all depends on the individual. If you sweat while playing, get a terry cloth wrist band (I mostly use it to mop up sweat off my forehead to keep the sweat out of my eyes). If you have sore or tired wrists, use a support wrist band.
