Black has many ways of playing, but including a6-a4 is useful to discourage White's plans of castling queenside in the future.
1. e2e4  1... c7c5  2. g1f3  2... d7d6  3. b1c3  3... g8f6  4. h2h3 According to my database, this is only the second time Vachier-Lagrave plays this system. It's interesting to see him avoiding a theoretical battle in his favorite Najdorf Defense.