Benko gambit occurs after Benoni defense. I think , it's the most reasonable and desirable  continuation of Benoni defense for black. Sacrificing a pawn black intends to put pressure both on white's kingside(after Ba6) and on queenside after(Rb8) , but in the Benko gambit blacks are taking risk , as with whites careful play the position will become favourable for white.

http://www.mediafire.com/watch/0qma8jlflrcjp3p/Accepted_benko.flv