The advantage of the BB30 standard over standard cartridge BBs is that the BB shell is lighter (no bearing cups; bearings are pressed-in direct fit) and the BB shell is larger, permitting bigger down tube, seat tube, and chainstays to be accommodated. The larger tubing better resists deflection due to pedaling forces (stiffer BB shell). Basically, BB30 is lighter and stiffer than conventional bottom brackets.

http://www.bb30standard.com/

EDIT- RacerOne beat me to it.