Well, a4 has another strong purpose here, simply based on the fact that the King's rook is on f8. (This is actually a very good example of a "Positional Consideration".)

Black's King appears to be in worse shape than White's. Therefore, White wants to exploit this by opening-up the Queenside, thereby weakening Black's King even more.

In my opinion, of course.

Oh, and welcome Glenn!