Remove the defender

Often the opponent defends against a threat with one piece, but that piece can be captured or threatened. If you can capture a defender, then whatever it was guarding will no longer be protected.

Black's king is defending both promotion squares for White's pawns. Which pawn promotion wins the game for White?

If we advanced the other pawn, it would have been taken by the King and the other promoting pawn would be under attack by the bishop.