This move looks harmless, but it's more interesting than it seems. The main idea is to prevent the ...Bb4 that is so typical of the Taimanov Defense. Besides, a3 ends up being useful anyway in many variations, since it prevents b5-b4.
1. e2e4  1... c7c5  2. g1f3  2... e7e6  3. d2d4  3... c5d4  4. f3d4  4... b8c6  5. b1c3  5... d8c7 It amazes me how Carlsen can play several different variations of the Sicilian as Black, all with equal skill. 6. c1e3  6... a7a6 