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...Qd5/8. c4

8...Qd6 is necessary to preserve the attack on the White knight. It's true that Black could have played 7...Qd6 in one move, but the move 8.c4 is hopefully going to prove to be a concession by White.

Horizontal queen moves like 8...Qg5 are inferior because the queen won't be hitting an undefended piece on b5 the way it would after 7...Qg5.