What is the idea behind playing 6.a3 in the Sicilian Najdorf?
[FEN ""] 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. a3
This seems to be getting popular at the highest level (e.g. yesterday/today's games at the Tata Steel Masters, Carlsen - Wojtaszek, Karjakin-Giri).
Is it just a waiting move or is there some deep idea behind it? As far as I am aware in most lines black is not aiming to put any piece on b4 (which would be prevented by 6. a3). In the poisoned pawn variation a3 prevents the capture Qxb2, but other than that, I don't see what white wants to achieve with 6. a3. Is he preparing to play b4?