To protect your king from the opponent's bishop (F1 > B5)

The computer suggest a6 because it restricts the white bishop. After a6 the best move for white is to fianchetto the bishop because there's no other place to put the bishop. It's also why after e6 the best move for white is to trade the bishop because the bishop is so bad.

But honestly if your opponent starts with Qf3 you're playing at a level where you most likely can't capitalize a6 vs e6 so what played is perfectly fine.

Now you can develop the dark square bishop and the queen.