White's d5 pawn is now attacked twice, he can defend it again with 5. Bc4 developing a piece. Other moves such as 5. Nf3 fail to 5... Bf5 when white must play 6. Bd3 to prevent the fork on c2. This now allows 6... Bxd3 7. cxd3 Nxd5 where material is even and white's pawn structure is significantly worse due to his doubled isolated pawns.