Why is trading a bishop for a knight the best move?

I am unclear why this is the best move.  I certainly didn't know what to play at the time, but I wouldn't have guessed this...

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You had 4 candidate moves:  Bxf3, O-O, Nd7 & c6; e5 is hanging, threatened by Nf3.  You chose least desirable move.  Why?

Bxf3 removes the threat, allows O-O & c6;  your good B for White's good N is good exchange.  Move is valued at almost 2 pawns in favor of Black over c6. 

While Nd7 supports e5, it poorly placed and cannot support c6

O-O & c6 allow Nxe5, where N is well placed and attacks c6

After your review of potential moves, Bxf3 should have stood out.  Work on making better moves.

I didn't notice the hanging pawn. Thank you!

Hello

Ur pawn is hanging that might be why

Yeah it's the pawn.

His knight can capture your pawn, and you could then take his bishop, but he would take it back and you would have a weak center

yea its probably a bug in stockfish anyone knows bishops are better than knights

also not only is it to get rid of the hanging pawn it improves your position the bishop was doing nothing since he un-pined himself by moving the bishop and his knight was controlling key squares in the center and bishops are not always better than knights in fact there are many positions where you would rather have a knight then bishop 

1. Your pawn on e5 is hanging so you're removing the attacker. Nd7 might fall to a future nxe5 bxg4 tactic. 2. The position is closed and removing the knight for the bishop gives you a clear domination of the very important d4 square. Your control of the center will be very strong as long as the center stays closed.

One reason may be because the white squared bishop is hampered by its own central complex of pawns on white squares. As black I would then consider h6 - to prevent Bg5 - with possible g5 expansion depending on  circumstances, with play on the dark squares.