[fen ""] 1. d4 d5 2. c4 c6 3. Nc3 Nf6 4. cxd5 cxd5 5. Bf4 Nc6 6. e3 a6
I do not see the point of this move. At first I thought it is aimed to prevent Bb5 and thus maintain the knight on c6, which controls e5, a square where White has a firm grip. It also prevents Nb5-c7, which is not yet a threat but might very well become one, as the White bishop is aiming at c7.
But after checking those threats aren't real and even if ...a6 is not played, in most continuations the best move for White is Bd3. Meanwhile the move ...a6 actually creates responsibilities (I would not yet say weaknesses, but work is required to avoid them) for Black, mainly because the advance ...b6 to control c5 leaves the queenside vulnerable (as demonstrated in Kramnik - Aronian 2012)