If a player would like to wait for opponent’s move to implement his plan, then a3 is the way to proceed in a game. Though the idea may also be to plant the white light square bishop on the a2 square when Na5 is played. Another option is also to go b4 and fianchetto the bishop on the b2 square.
We can observe in the game what is white’s plan after playing a3 in the below game:
1.e4 e5 2.Nc3 Nf6 3.a3 Bc5 4.Nf3 d6 5.Bc4 Nc6 6.d3 h6 7.Na4 Bb6 8.h3 Be6 9.Bxe6 fxe6 10.0-0 0-0 11.c3 Qe8 12.Nxb6 axb6 13.Qe2 Nh5 14.d4 exd4 15.cxd4 e5 16.d5 Ne7 17.g3 Qg6 18.Kh2 Rf6 19.Nh4 Qf7 20.Be3 Rf8 21.Rac1 c6 22.dxc6 bxc6 23.a4 c5 24.Qb5
Major black activity is on the kingside and white jumps ahead towards the black’s queenside.
g5 25.Nf5 Nxf5 26.exf5 Rxf5 27.Qxb6 Nxg3 28.Kxg3 Rf3+ 29.Kg2 Qf5 30.Qb3+ Kh8 31.Kg1 Qxh3 32.Qc4 Rg3+
An error by black which further leads to a draw
33.fxg3 Qxg3+ 34.Kh1