Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. Nc3/4...fxe4/5. Nxe4/5...d5/6. Nxe5/6...dxe4/7. Nxc6/7...Qg5/8. Qe2
Black has to defend the e4-pawn, and does so with 8...Nf6, which has the additional benefit of covering h5.
8...Bf5 has a specific tactical problem: 9.f4! is a counter-sacrifice that White wants to play anyway, and with the bishop on f5 the pawn can't be taken: 9...Qxf4?? 10.Rf1 Qg5 11.Rxf5 Qxf5 12.Nd4+ is curtains. Plus, in the current position Black does have a secondary threat of ...a6 and ...Bd7 pinning and winning the c6-knight, which doesn't work if the bishop is away defending e4. Plus, f6 is almost certainly the best square for the knight anyway.
Another line that doesn't work is 8...Qxg2. The downside of playing ...f5 in the opening is the need to be vigilant about queen checks on h5. Here 9.Qh5+ g6 (or 9...Qg6 10.Nd5+) 10.Qe5+ punishes Black severely for her pawn-grabbing. 8...Nf6 cuts out this check, turning the illusory threat of ...Qxg2 into a real one.