|White to move|
As you see, both a white and a black bishop are hanging. Taking the black bishop is just a trading off of the bishops. But 1.Bxc6 gains two tempi. It attacks the black rook, and it prevents the bishop from being taken on e4. So even one tempo is a defensive one! The move trades off a problem piece of white with tempo. It is important to develop a sense for those two tempo moves. Both offensive and defensive.