A single bishop is forever confined to one color, but a pair working together can control the entire board. In open positions with pawns on both wings, the bishops' long range lets them dominate knights and create threats on opposite sides simultaneously. The classical estimate is that the bishop pair is worth roughly half a pawn.
The practical lesson is to value your bishops in open, fluid positions and to avoid trading one of them without good reason. Conversely, the side without the pair seeks to keep the position closed, fix pawns on squares that blunt the bishops, and trade one of them off at the first good opportunity.