Why would I trade bishop for knight?

The computer gives Bg4 and after h3, Bxf3. I have seen this a couple of times now in similar positions, but I'm reluctant to go for it as I don't see why it would be good for black.

Any help appreciated.

9...Bg4 10. h3 Bxf3 11. Bxf3 Nd7 12. Ne2 Qb6 13. Rb1 cxd4 14.exd4 Nxd4 15.Nxd4 Qxd4 16.b3 loses the bishop's pair, but wins pawn d4. Another idea is 9...a6 intending 10...b5.