This is a position from a game between Fischer and Ibrahimglu, 1970.
Black's last move was 11. ... Qc7 from d8. According to Stockfish (depth = 28), 12. a4 is the best move for White. The other recommended moves are Nb3 and Rd1. But a4 seems to be better than Nb3 by more than 0.1 point.
It seems to be a very logical move as Fischer played it too. Only the logic behind it is out of my (an intermediate player) reach. :)
The game continued with 12. ... Rad8 13. Nb3 b6 14. Be3 c5... Fischer went on winning this game.