Nc3 keeps options open against ...e6, ...g6, and ...Nc6
2.Nc3 prevents ...d5 for one move
f4 can follow once the knight controls d5
Playing 2.Nc3 before 3.f4 gives White flexibility. Against ...e6 (French-style), White can still play f4. Against ...g6, the GPA setup with f4 is ideal. The knight also eyes d5 and e4.