Position Question

rnr3k1/2qbbp1p/5npB/p2pNp2/3P4/P1P3N1/2B3PP/R2Q1RK1 b - - 3 20

I  came across this position while studying the Hillbilly Attack in the Caro-Kann.  It is Black to move and the engine (Stockfish 14 depth 30) says that the best move is Bf8.

I understand that White's Bishop on h6 is annoying, but it seems to me that Black should want to keep the dark-squared Bishop.  Black has a command on the light squares, so the dark-squared Bishop should be essential.

I am certainly missing something here, so perhaps someone can explain to me why I would want to trade what looks to me to be a good Bishop for what looks to me to be a bad Bishop.

Thanks - 

george

Black is a pawn ahead. White has some attacking prospects against the black king, for example ...Qxc3? Bxf5. Thus black should trade pieces and approach a won endgame.

I guess I would have thought that forcing the Queen to do the dark-squared Bishop's job would be a high price to pay for trading down.