Does anyone really play 9 a4 against you? It's a rare move in practice - White more often tries 9 h3 to avoid the pin, 9 d4 disdaining the pin, or 9 d3 heading for less critical variations.
a4 is a useful move for White in many lines, but not this soon - except in the case of 7...0-0 8 a4 where it is designed to avoid the Marshall Gambit.
Ahhh you misunderstand, I am trying to learn to play the opening as white. So I have studied a bunch of games and over and over again I see this move a4 at some point, often quite early in the opening and I am left wondering why it was played. I wondered if it was some common positional theme rather than a tactical ploy that is going over my head.
I have noticed some games in the closed variations of Ruy Lopez where white plays a4. It seems to be quite a common theme in many lines. I have seen it in the Archangel and the chigorin amongst others. Clearly it has some immediate tactical threats in that it threatens to win a pawn, but GM's don't make moves like this just to threaten an easily defended pawn (black just has to defend his a8 rook). There must be more sound positional reasons behind it, but I don't really see it. It just looks ugly and weakening to me.
Here is an example in case you are wondering what I'm talking about.