In the position you posted....... I think White has better moves here than a4.
You are right: d3 and 0-0 are better!!! a4 doesn't do much here.
In General, the move a4, should not be heavily considered in the opening because: piece development takes priority over that and it is not a threatening move, capturing move, protecting move, flight move, trapping move, or blocking move. (I personally developed these Concepts), which can be applied here.
I hope this helps!
Sincerely,
dustinkinney.