What are the pros and cons to giving up a knight and Keeping the Bishop pair.

There are cons to giving up the knight as well, but the presence of a "knight pair" doesn't really mean anything, but the presence of the "bishop pair" is worth something; the bishop pair is about half a pawn advantage if the player knows how to utilize it.

Some positions have bishops better and some positions have knights better. There are many factors to consider and IM Jeremy Silman has written a lot (as well as others) on this "eternal battle of bishop versus knight." As a really broad generalization, knights will be better in chess positions with a closed pawn structure and bishops will be better in chess positions with an open pawn structure.

You already have a rapid rating in the 1400s though, so you probably got there by knowing some basics like this. What you might instead be running into is trading off pieces (knight or bishop) of equal material, but exchanging off pieces which are developed or long-term good pieces and this is why the engine considers the move an innaccuracy (not just because it was a bishop or knight).

If you have specific positions to show as examples, then we can help in this thread to clarify which piece we believe is preferred there (bishop or knight).

This is one example of a blitz game I played on lichess... I was white.. stockfish told me moving the bishop to d2 was inaccurate and wanted me to move my knight to c3.

Good position, but the reason for the computer choice has nothing to do with the "bishop pair" because 10. Bd2 allows for both players to lose a bishop. That's just an equal trade in that regard. Black did develop with a check though and this helps them castle quickly if desired.

The reason I suspect 10. Nc3 is better is because it develops a piece and black doesn't have an easy trade of equal value because 10. Nc3 ...Bxc3?! 11. bxc3 gives white a strong pawn center with pawns on c3, d4 and e4. White has the amazing center thanks to the pawn duo and the extra support from the c3 pawn.

Keeping the bishop pair can be a strong strategic advantage, especially in open positions where bishops control long diagonals and can dominate both sides of the board. They work well together, covering both color complexes and often shine in endgames. However, giving up a knight has its drawbacks, especially in closed or cramped positions where knights can maneuver better around pawn structures. Knights also excel in tactical situations, like delivering forks and controlling key squares that bishops might struggle to reach. Ultimately, whether to give up a knight depends on the position and how open or closed the game is likely to become.