Why does f3 weaken white's position here?

Question

Take this game:

[fen ""]
[Startply "13"]

1. d4 d5 2. Bf4 Nc6 3. Nf3 Bf5 4. e3 Nb4 5. Nh4?? Bxc2 6. Qd2 e5 7. Bg3 Be4 (7... Bxb1 8. Rxb1) 8. f3 Bxb1 9. Rxb1

I'm specifically curious about the difference between

1. 7... Bxb1 8. Rxb1 and
2. 7... Be4 8. f3 Bxb1 9. Rxb1.

Using Stockfish 14+ at depth 22/22, the engine thinks that the first option is -0.9 and that the second option is -2.6. The only difference between these two positions is the f pawn. So basically stockfish thinks 7...Be4 can "trick" white into pushing the f pawn to f3 which drastically weakens the position.

But I don't really see why the f3 pawn weakens white here.

Answer 1

f3 is actually sort of forced due to black's annoying and powerful bishop on e4.

After that was taken care of, white now had a knight that cannot go anywhere! The f3 and g2 squares are covered! Plus the bishop on g3 prevents any push or advance of the g2 pawn to make space for the knight. Sure you can play f4 to attack while creating space for your knight, but in doing so black can just trade his pawn to cause white to have 2 backward pawns!

Answer 2

White's troubles started with 5. Nh4, which is an insufficient answer to 4... Nb4. The evaluation that you quote, -2, stems from this.

7... Bxb1 is likewise bad, allowing white to consolidate to a certain degree. Among the alternatives, 7... Be4 is good for the reason stated above, hindering whites developement and asking white to chose a lesser evil.

The inclusion of f3 also crucially takes away the flight square for the misplaced knight, and black should continue with 9... Nh6, further reducing the h4-knights options and keep threatening g5. After the dust around black's own weakness, the uncastled king, has been settled, black has a dominating position.

So, black's dubious 7... Bxb1 allows white to at least bring the knight back into the game with Nf3, or Nf5 even, as the e5-pawn is hanging. There, black has other priorities that give some breathing room for white.

Also, note that after 8. f3, black's best move is still 8... Nc2+!

Answer 3

f3 is not weakening, it just takes away a square from the h4-knight (which in turn forces the g3-bishop to remain in place to defend it).

Still, it was kind of forced - Black currently threatens ...Nc2+ and although White can defend against that threat without playing f3, the bishop on e4 is also stopping White from developing the f1-bishop / castling, and White's pieces on the kingside are very vulnerable to Black's pawns (e.g. 8.Na3 h5 9. Nf3? h4 leads to a bishop trap).