Why should I sacrifice Bishop? I had a game with a friend, I obviously lost, but now I don't understand why, while reviewing my game, the CPU suggest that at this point the best move is Bishop in b5.

If the Queen decides to take the Bishop, there is nothing taking the Queen back:



Even continuing the game from there against CPU, the CPU doesn't take Bishop in b5. I don't get why.
When ... Qxb5 is played, White can play Bxd6. This is a great spot for the bishop, because now your opponent can't castle and your bishop is controlling some nice squares. If Black doesn't capture on b5, then he pretty much needs to move his queen, since you're threatening it with that bishop. Once he retreats, you have another piece developed, while your opponent has wasted a move running away.