It absorbs moisture. Regular grips for me become slick when they get sweaty. That's why I use them anyway. I always used leather and just made the switch to synthetic and just kept over gripping like I always do.
Yeah, overgrips are pretty much for moisture management. I actually prefer the feel of a nice, tacky replacement grip like Gamma Hi-Tech, but no replacement grip absorbs moisture and stays tacky when your hand sweats like a good over grip will. If one has dry hands I think you could use whatever you want.
I put good overgrips in three categories:
1. Extremely tacky with so-so moisture management. Some of these (Gamma Razor Tac) will stay tacky and last a long time because their polyurethane top layer is thicker and rubberier. Some people use these only in the winter when their hands tend to stay dry.
2. Moderately tacky with good moisture management (Yonex Supergrap and Wilson Pro, and clones of same). Good durability, but eventually the thin polyurethane membrane that's the secret of these grips starts to tear. In generally, the secret sauce combo of a tack, durability, comfort and moisture management.
3. Moderately tacky only when wet, with excellent moisture management (Tournagrip, Solico Heaven, various Tourna clones and imitators). Tourna has notoriously poor durability because the polyurethane membrane is extremely thin, wispy, and discontinuous to allow sweat to immediately wick into the absorbent under layer. This discontinuous membrane quickly gets torn up by friction with the hand. Solinco Heaven seems to have solved this problem by using a thicker, more robust PU membrane that is perforated. Sweat goes into the underlayer at the perforations, while the thicker membrane in between the perforations holds up longer to friction than Tourna's gossamer membrane.