A variant of my own. The more modern line is Qb6.

White probably needed address e5 after the inevitable fall of the c3 bishop.

Trade down and get out with the win. There was of course more material to be had if I wanted it, but with an uncastled king and a material advantage -- get to an ending quickly.

I want to encourage the trade down - and I want to keep whites pieces difficult to activate, hxg7 doesn't bother me as it opens up a file. I'd rather activate my king than worry about the light squared bishops activity, and I'd very much like to pin the rook on a1 if I can possibly do that (leaving the knight on c4 stops Nd2).

I'm not overly interested in Rxf6, because the white king is out of the game as long as I leave the Rook here. Now to limit the queenside.

The pin on the knight is effective - and the d7 bishop doesn't need to go anywhere. I'd like to activate my king so I can march my passed d pawn. f6 needs to fall for that to happen.

the d7 bishop becomes very active...without moving.

I've taken away every meaningful square Its just about zugzwang. Anything white does makes things worse... massive material deficits will do that.

If white leaves c4 the d pawn queens. If white allows me to play a4 and doesn't abandon c4, then the a pawn queens. and the white king cannot make a difference.

truly this is unnecessary, but it gets the point across.