To keep crud and water from getting into the bearings via the down tube, seat tube and chain stays. Surprisingly a large amount of crud and water works its way into the BB, beer can sleeves are bad option because they dont allow water to drain out the BB shell, that is assuming your BB has a drain hole. Campy sleeves suck for this very reason to, they ride tight agioanst the wall of the BB shell and hold moisture in the tubes thus promoting rust.