Question about continuations of the slav defense

So I was playing around a bit in the Game Explorer, trying to work on transitioning my openings to middlegames (my middlegame is usually where my better games fall apart), and I noticed the following continuation of the Slav defense:

I know it's not an incredibly common way of doing things, but 5. A4 is the most common move for white in that situation. My question is: what exactly is white trying to accomplish by pushing the A pawn so early?

To prevent black from playing b5 so white can win the pawn back after e3 and Bxc4

Thanks guys, it's very obvious now that you mention it :). I love letting that poor little pawn hang in the balance until I choose my moment.