Preparing a pawn storm on the queenside with b2-b4 and a2-a4. 1.a3?! is less precise, since after1...f6 2.b4 Nf7 White will have to spend one more tempo protecting the b-pawn before he can push a3-a4.
Reinforcing the knight and preparing Qc3, d3-d4 and c4-c5 with a further attack.