Above my pay grade, but John Watson just finished off an amazing 14-part video series on the king's indian defense on ICC.
It looks to be like it is a move that is preparing for black's eventual f5 push. With the rook on a3 instead of a1
1) It is no longer on that all-important KID diagonal and being indirectly attacked by the bishop
2) It protects the c3 knight from discovered attacks on that diagonal
3) It provides more cover to some of the really important internal holes in white's position, especially e3
4) It could theoretically provide some defense along that third rank far down the road during a kingside pawn storm attack from black.
Basically, the idea is that white has a major long-term static advantage due to his pawn structure, so black is going to have to prove some dynamic activity to justify his position. Ra3 is aimed taking a ton of the sting out of two of black's most important dynamic advantages: the long dark diagonal and the internal weak squares in white's position.
Inb4 someone tells me how wrong i am