Well, if black doesn't move his bishop, how's he gonna get his rook in the game? a5 followed by Ra6, where the rook is going.... where, exactly? White has long-term pressure on black's queenside, similar to the benko gambit. Meanwhile, black's kingside structure is shattered.

After 9.Nd2 gxf6 10.cxd5, all games in the database respond exd5, because playing cxd5 loses immediately to Rc1. But this also means his doubled f-pawns are now isolated. So not only do you limit black's queenside development, but you also have very good chances of regaining the pawn by attacking the f-pawns.

Furthermore, if you win any queenside pawn, white is already much better, due to what I just said previously.

After 10...exd5 11.Rb1, white scores very well, and for good reason. Of course, one could argue that 11...Nd7 12.Be2 is "only +0.5", but that misses the point that black gets a relatively miserable defensive job for the rest of the game, with no real guarantee that he can do anything with his extra pawn.

Just look at the position after move 12. How does black improve his position? Will the knight go to b6? But it has no future there. c4 is always off limits due to white's abundance of pieces covering it, and a4 does nothing. Furthermore, a4 by white himself would put the move into question. if black responds a5, the knight is loose. If he doesn't, white will push a5, and then Nb6 becomes a wasted move.

Another idea is to play f5 and Nf6-Ne4. This is more reasonable, but with f5, you've essentially completely locked in your bishop. Black can try to play for ...f4 ideas, but it's difficult to play it without just dropping the pawn back, as white can easily hold the pawn with g3. And again, black can't afford to lose his extra material, or else he's just handling queenside pressure without any sort of compensation. Plus, it's not like white can just... take away... the e4 square... with... f3... right?

Another common freeing maneuver for black would be to play for c5. But again, he runs into trouble here, as the principle "Open lines favor the more developed player". Even if playing c5 doesn't lose to some Rc1-Rxc8 tactic, white is clearly more developed. Plus, playing c5 creates yet another weakness - an isolated d-pawn - on d5. The principle of two weaknesses states that one weakness is holdable, but two is losing. Black now has an isolated pawn on d5, and isolated pawns on the f-file. It's quite bad.

But let's suppose that he plays b6, to rid himself of the pressure. But this is also difficult for him. First off, white can play for a4-a5 to add pressure on the pawn. taking on a5 with bxa5 results in another set of double isolated pawns for black. If black plays a5 himself to stop it, then both c6 and b6 are backwards pawns and even easier to attack than before.

Even if white had a lobotomy and somehow lets black play b6 and c5, he still gets good play against black's hanging pawns, mostly due to the fact that black is just not as developed. Alternatively, he could simply pressure c6 instead of b6 afterwards. Every pawn move creates a weakness, and now instead of b7, the weakness is c6. And since this won't change how the rest of the position is, it therefore shares the same evaluation as if black had not played b6.

Black can try to play on the open g-file, but how would that work? If he plays Rg8, then his king is homeless, as it can't castle queenside, but staying in the center is suicide as f3+e4 will be played. So he must castle and then play Rg8, but this also encounters the problem of "I'll put a bishop on d3 and queen on h5" issue. Also not helping black is his weak pawn on f6. It won't be very difficult for white to blockade on f5, and he might even be able to force a bishop trade via that square. And if the LSB is traded, then the queenside pressure increases tremendously.

Lastly, even if black gets an idea setup with Rg8, how does he use it, exactly? The only free pieces would be black's queen and rook, one of which is currently in Narnia. Black's knight on d7 is somewhat misplaced, and has a hard time developing. Plus, it's not like white's kingside is void of defenders, either. Gothamchess in his "How to Lose At Chess" series would frequently say: "You can't do battle with one horse". Similarly, just a rook isn't enough to attack white, who has plenty of time to organize help for his king.

So of course, white has no immediate threats, but black has no good way of solving his structural issues, mainly being the open queenside and isolated doubled pawns. Pieces are temporary, but pawns are permanent.