The whole opening is questionable for black, giving white a big advantage for nothing. 3..f6 is ridiculous, leading to 4. d5 and the retreat of the black knight to its home square.
I can't believe you have seen it very often. I sure hope that you weren't black.
When black gives white so much in the opening, white seems almost ready to pounce with Nxe5 and Qh4+, 8 a3 has to be questioned. What is white worried about?
I think that 9 Bb5 isn't as strong, because it doesn't go directly after the king as 9 Bc4.
At any rate, in order to understand more, you will have to spend some time running through the various engine moves.
1. e4 Nc6 2. Nf3 e5 3. d4 f6 4. d5 Nb8 5. Be3 c6 6. Nc3 cxd5 7. Nxd5 Nc6 8. a3
d6 9. Bb5 Bd7 10. O-O Nd4 11. Bxd7+ Qxd7 12. Nxd4 exd4 13. Qxd4 Ne7 14. Rfe1
O-O-O 15. Nxe7+ Bxe7 16. Qxa7 f5 17. Bb6 Rde8 18. Qa8# 1-0