Hello, I've got a question.
In this position, engine suggests Bb8 as best move. Has anyone got a clue why?
Why Bb8?
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Well, I'm not sure there is a single reason, but lets work through it:
(1) the plan is bb8 qd6 largely forcing f4 (g3 being worse). F4 is at best not a helpful move for white, and actually looks weakening in this position. How does white free up the rook to move? With so many pawns on dark squares, how does white get any future at all for his dark square bishop? I don't have a specific sequence in mind to exploit it, but white's position certainly looks quite bad.
(2) bb8 qd6 links the black rooks. How else can that logically be done? Qd7 runs into bb5. Stockfish is very unimpressed by Qb6. Not so much because of na4 like I was thinking, but it seems more so because it doesn't do much else besides link the rooks, which I guess isn't as much of a useful factor as I was thinking, because what does it actually allow black to do? Ok, so lets discount linking the rooks and move on.
(3) in terms of other candidate moves that actually do something...there isn't much. A5 is something that looks logical, and should be good. Apparently, b5 is a bad answer to a5 because it frees the d7 square for black's queen, and the bishop + queen battery is dangerous with no knight to defend over there. So after a5, stockfish has white playing pxp in which case qxp looks nice, black has improved the position some, even if it isnt an overwhelming change. Another possible move is Ne5, which actually doesn't drop the dpawn, but how does trading knights help black? The white knight is in an awkwards spot blocking the bishop and the cfile.
Ok, so really comparing bb8 to a5 is a hard positional issue. Maybe a stronger player can offer more insight there. But as a human, those are the two moves I would be focusing on because they do something to change the position in black's favor without fixing any of white's problems.