In an endgame with only the bishops and pawns, opposite-colored bishops are famous for their drawing tendency: even an extra two pawns is frequently not enough to win, because the defending bishop blockades on its color and the attacking bishop simply cannot challenge it. Knowing this saves half-points in countless rook-less endings.
The paradox is that in the middlegame the same bishops favor the side with the initiative. Since the defender's bishop cannot guard squares of the attacker's color, an attack against the king on those squares can be overwhelming — effectively a piece up where it matters. The evaluation thus flips entirely depending on whether other pieces remain.