You are better discussing this in the general chess discussion forum. A classic example is the Ruy Lopez exchange. 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Bxc6
While doubling the pawns gives white a better pawn structure and better chances in the end game (they can create a passed pawn), time has told us that black gets compensation with the bishop pair.
A bishop is usually slightly more powerful than a knight, but not always – it depends on the position. Some engines give the value of 3 for the knight and 3.4 for the bishop.
Some reasons for this evaluation are:
-Bishops are stronger than knights in open positions, and open positions are more likely than closed ones. If you think of the endgame in particular, many pawns have been exchanged, i.e. many end games are open, favouring the Bishop
-The Bishop pair is an advantage, as the light square bishop covers the squares the dark square bishop doesn't and vice-versa. So 2 bishops together compliment each other, but the 2 Knights do not. They actually replicate each other, potentially making one of the Knights rather redundant
-In an endgame, a Bishop can cover both the Kingside and Queenside because it is a long distance operator, meaning it can perform offensive and defensive duties at the same time (e.g. help push a friendly passed pawn and stop an enemy passed pawn at the same time in a race). The Knight cannot do the same