Why are these moves recommended? I am playing chess on the chess.com app with the top three suggested moves:



I am an amateur. I get the basic idea that developing your knights is a common, good thing early in the game, but why is that specific pawn move a7-a6 recommended? The queen could take it, but it’s protected by the rook.
The point is that 4...Nc6 could be met with 5.Bb5, when the knight is pinned and it's likely White will exchange it off with Bxc6. Without the c6-knight, Black will lose some influence over the key d4- and e5-squares. Playing ...a6 before developing the knight aims to prevent this. 

Of course there's a trade-off here, since Black is spending a tempo just to prevent a relatively innocuous exchange.