Putting the bishop on a5 is good because it controls d8, preventing your rooks from moving to the open file. See your two rooks staring at the a7 and h7 pawns? They need to move to more active squares, your opponent is trying to stop you from doing that. The reason you lost this game is because instead of thinking "where can I put my worst pieces so they are more active?" like you should be in positions like these you pushed some pawns on the other side of the board. You should always be thinking, "can I improve my pieces."