What is the idea behind playing 6.a3 in the Sicilian Najdorf? What is the idea behind playing 6.a3 in the Sicilian Najdorf?

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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?
The move a3 serves basically two functions: 


White can develop the bishop to c4, and keep it aimed on d5, even if Black attacks it with b5, or, as Giri did, with Be6. In the lines where Black plays ...e5, the battle for d5 is often very important so it could be worth spending a tempo in it.
Black often plays b5 to gain some space on the queenside. The move a3 then discourages a further advance of the pawn to b4.


Another reason to choose this line could be that the main line is heavily analyzed, beyond the 20th move, and the White players didn't want to engage in a theoretical battle. This would especially apply to Carlsen who often plays 'sidelines', and this tweet from Karjakin confirms it as well:


  The only reason why I played 6.a3 against @anishgiri is that I forgot adaptor for my laptop and I urgently had to create a new ideas


Thanks @DagOskarMadsen and @DavidRicherby for drawing my attention to this tweet.