By striking immediately at the center, White wants Black to play ...e6, locking in the light squared Bishop. Black can avoid this for one move with 4...Nf6, but starts to run out of options after 5.Nc3. Developing ...Bf5 before playing ...e6 is not good because it weakens the light squares on the Queenside, starting with b7.
Black can bite the bullet with 5...e6 6.Nf3, followed by 6...Bb4, 6...Be7, or 6...Nc6. Keeping the diagonal open with 5...Nc6 runs into 6.Bg5, and Black has the same problem to find an alternative to 6...e6. Here the move 6...Ne4? is not an option because of 7.Nxe4 dxe4 8.d5.
An alternative to 5...e6 is 5...g6. Black is willing to sacrifice the d-Pawn with 6.Qb3 Bg7, feeling confident that White can't keep it.
