The idea would be to first use your superior Knight to gain an advantage, before allowing the position to open up to the point that the Bishop is better.
Knights are better in closed positions, but what about later?
Knights are good when the game is closed and once it opens up, Bishops are good generally . This is the rule of the thumb. However nothing cannot be generalised, it changes as per situation.
Ideal pieces are pieces that can potentially come into contact with weak points*, are pieces that control many squares, and are protected pieces that can't easily be chased away from a good location.
*Both yours (for defense) and your opponent's (for attack) count.
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Like blueemu said, when your position is better, that means when you transition it will be to another good position. There are lots of different advantages in chess, so having knights vs your opponent's bishops in an open position doesn't necessarily mean anything.
I've read that knights are stronger than bishops in closed or cramped positions, but does that mean you should favor trades where you give up a bishop for your opponent's knight? Don't most positions open up eventually, and then wouldn't you regret giving up your bishops?