In the London, after 1.d4 d5 2.Bf4 c5 3.dxc5 Nc6 4.Nf3, why is "blocking your bishop" the best move

Question

In this situation:

[FEN ""]
[StartFlipped "1"]
[StartPly "7"]

1.d4 d5 2.Bf4 c5 3.dxc5 Nc6 4.Nf3

Stockfish 16 really does not value getting the bishop out (Bg4 and Bf5 are not the "best" moves) which is slightly contradictory with "standard" rules of thumbs. I know rules of thumbs are to be broken in certain situation but I was wondering here what intuition there was behind e6 (the Nf6 move encourages e6 too down the line) so that I could sharpen my understanding of playing against the London. Is it because if I don't protect the d5 pawn they can play Nc3 and double the threat they currently have on my d5 pawn? It doesn't seem to be the plan Stockfish takes for White when I play Bf5 though (Stockfish 16 advocating 5. c3 after 4. ... Bf5).

Answer 1

e6 is really preferred so you can take back the pawn on c5, which can't be defended at the moment. If white is given one more move then the c5 pawn can be defended, with for example c3 then b4. If white is allowed this setup then black's queen side is a wreck (white queen on a4 is a lot of trouble for black) and black will never get to castle either side.

On the other hand, black's light square bishop doesn't have a lot to do if moved to f5 or g4, it's better to put it on d7 to be moved to b5 later (being supported by A pawn obviously)

Answer 2

Don't think of it as blocking the bishop. Think of it as regaining the pawn on c5. If you develop the c8-bishop first, White has moves like Nbd2-b3 or c3-b4, defending the pawn and making it hard for you to regain the sacrificed material. 4...e6 regains the pawn immediately, unless White does something drastic like 5. Be3, which is very ugly (blocks the e-pawn, moves an already-developed piece, etc.).