Knight for knight is three points for three points, and the scoreboard does not move. The board does.
7
2
a
e
g Nothing was won there, and yet White would play it in most positions and Black would avoid it in most positions. That gap between the arithmetic and the position is what trading is actually about.
What changes when a piece comes off
Four things, and you can check them in the time it takes to play the move.
Material, obviously: is it really level, or am I giving a bishop for a knight, or a rook for a bishop, which is the exchange?
Pawns. Recaptures move pawns, and pawns do not move back. In the diagram Black’s kingside goes from three healthy pawns to a doubled pair and an open file next to his king.
Space and air. If your pieces are stepping on each other behind a wall of pawns, every trade helps you: fewer pieces need less room. The player with more space wants to keep pieces on for exactly the same reason.
Kings. Trading queens usually ends an attack. If he is the one attacking you, the queen trade is the best move on the board even at the cost of a pawn.
Who wants it
The side that is ahead. A piece up, trade everything you can and let the extra piece decide the endgame: that is simplification, and it is how most winning positions actually get converted.
The side that is worse, generally, does not. Behind in material, you need pieces on the board, because pieces make threats and threats make mistakes. Trading your last rook when you are a pawn down is often the moment the game stops being a game.
And there is the personal version. Trade off his good pieces and keep your own. If his bishop is raking your king from b2 and yours is buried behind a pawn on d4, then trading bishops is a good deal for you whatever the point values say.
Before you take, ask three things
Does he recapture, and with what? What does his recapture attack after it lands? And is there a better move than taking at all, a check or a capture somewhere else?
Most beginner trades happen automatically, by reflex, and reflex is what the counting on the piece values page is meant to replace. Two seconds a capture is enough. Half the trades you would have made on autopilot turn out to be the trades your opponent wanted.