Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...Nf6/6. Nxf6/6...Qxf6/7. Qe2/7...Be7/8. Bxc6/8...dxc6/9. Nxe5

9...Bf5 is a useful developing move. 9...O-O is also a useful developing move.

Why is ...Bf5 more popular than ...O-O? It comes down to a general principle of opening theory: postpone the hard decisions for as long as you can. Black in this position no longer has to decide where to put the c8-bishop. The White knight is hitting d7 and g4, and on e6 the bishop will be vulnerable to tactics like Nxc6 as soon as White gets a rook to the e-file. So it has to go to f5. That's an easy decision. Whether Black wants to castle queenside or kingside is a hard decision. Make the easy decision first!