user1375: In this position (White to play)

(Picture credits to the chess compass extension that analyzes a chess.com game after it ended)
The current Stockfish evaluation (before White plays) is ~0.0.
But SF would give a -0.8 (+0.8 for Black) if Bg5 takes Nf6 (playing around with SF, the rating goes from -0.9 [picture] to a max at -0.3). Next, SF recommends to take the B, gxf6.
Why would that be?

Is it because a B is usually worth more than a N?
or in this particular position B had better opportunities rather than take the N?


user20688: You seem to have made some random repetition moves Ra1-Re1 and Ra8-Rb8 before this position, so that Ra1 would be a draw as indicated by your graphic. This makes no sense otherwise, because Ra1 hangs the knight on e2. If not for the previous moves that lead to threefold repetition, the real evaluation of this position is that black is much better (more than -1 advantage, possibly winning) even though Bxf6 is the best move.
Why is Bxf6 the best move? This is an endgame (or late middlegame), and Bxf6 doubles blacks pawns and isolates the h pawn, both of which can become weaknesses for you to attack. Possibly even more important than this is that the black knight has prospects to jump to e4 or d5 and wreak havoc on white's position, while the white bishop is awkwardly placed blocked by white's own pawn on f4. Thus, this is a trade of a bad bishop for a good knight which is a good idea for white.
Nevertheless, white is still worse because black has the bishop pair and more space on the queenside. White's pieces are placed somewhat awkwardly, especially the b3 knight which is trapped by black's pawns and unable to advance forward. The knight on e2 blocks white's control of the e file, but cannot easily be moved without hanging the f4 pawn. The piece coordination is nonexistent in white's camp, but perfectly harmonious for black.

1375: <p>In this position (White to play)</p>
<p><a href="https://i.stack.imgur.com/UzG50.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UzG50.png" alt="enter image description here" /></a><br>
<sub>(Picture credits to the <a href="https://www.chesscompass.com/" rel="nofollow noreferrer">chess compass</a> extension that analyzes a chess.com game after it ended)</sub></p>
<p>The current Stockfish evaluation (before White plays) is ~0.0.</p>
<p>But SF would give a -0.8 (+0.8 for Black) if Bg5 takes Nf6 (playing around with SF, the rating goes from -0.9 [picture] to a max at -0.3). Next, SF recommends to take the B, gxf6.</p>
<p>Why would that be?</p>
<ul>
<li>Is it because a B is usually worth more than a N?</li>
<li>or in this particular position B had better opportunities rather than take the N?</li>
</ul>
 20688: <p>You seem to have made some random repetition moves Ra1-Re1 and Ra8-Rb8 before this position, so that Ra1 would be a draw as indicated by your graphic. This makes no sense otherwise, because Ra1 hangs the knight on e2. If not for the previous moves that lead to threefold repetition, the real evaluation of this position is that <strong>black is much better</strong> (more than -1 advantage, possibly winning) even though <strong>Bxf6 is the best move</strong>.</p>
<p>Why is Bxf6 the best move? This is an endgame (or late middlegame), and Bxf6 doubles blacks pawns and isolates the h pawn, both of which can become weaknesses for you to attack. Possibly even more important than this is that the black knight has prospects to jump to e4 or d5 and wreak havoc on white's position, while the white bishop is awkwardly placed blocked by white's own pawn on f4. Thus, this is a trade of a bad bishop for a good knight which is a good idea for white.</p>
<p>Nevertheless, white is still worse because black has the bishop pair and more space on the queenside. White's pieces are placed somewhat awkwardly, especially the b3 knight which is trapped by black's pawns and unable to advance forward. The knight on e2 blocks white's control of the e file, but cannot easily be moved without hanging the f4 pawn. The piece coordination is nonexistent in white's camp, but perfectly harmonious for black.</p>
