bxc5 wins a pawn on the spot. After Bxc5 white replies Nxe5 forking queen and bishop and any pressure black had on the kingside is gone. With the bishop attacked 3 times (by knight, bishop and queen) black is forced to play Bxe2. 

Add to that the fact that the b7 pawn is also en prise although white has to bear in mind Bb6 cutting off the rook's retreat.

    [fen "rn3rk1/pp3ppp/3b2q1/2p1p3/1PP3b1/3P1N2/P3BPPP/1RBQR1K1 w - - 1 1"]

    1. bxc5 Bxc5 2. Nxe5 Bxe2 3. Rxe2