Why 23. c4?

This is a game I played earlier, and my blunder was not playing c4. Why was it so imperative I played it? I didn't even consider it during the game. Activate the dark square bishop maybe? Look to pin the knight to the rook take the knight and double opponents c pawns? This is just speculation anyway. Any feedback is welcome. Thanks for reading.

Only looking quick: It was not imperative playing c4, it was important to avoid c3. Cause on Bg5 your queen has no good retreat, look at the game. Maybe Rd1 instead is even better than c4.

I'd say 23...c4 is recommended so that you can vacate that square and put your dark squared bishop on the same diagonal as white's queen and king. That would also build up pressure on f2, with your rook on f8 bearing down (eventually). The blunder on capturing the bishop on d5 is that it leads to trading off your knight, but more crucially, your dark squared bishop, which is a valuable attacker if it can get to the a7-g1 diagonal. Just my two cents!

Yup, now I see it. c4 is a clearance move for the bishop. Shows how much I still have to learn as I hadn't even considered it as an option. Thanks for the responses, I really appreciate it! @royalblue04 @domprocopio

Ouups, you were black... sorry.

The fault was mine. I accidentally posted from whites perspective! @royalblue04

Well white ends up with a powerful knight after Nxd5. Fortunately for you, white blundered right after.

The computer still recommends ...c4 at the end of some variations I looked over, and I think it's about keeping the b pawn backwards and the a pawn isolated, too.

preparing bc5 or bb4
and not 23.c4
23...c4

23. ...c4 opens up the a7-g1 diagonal for your dark squared bishop, at the same time making it difficult for White's bishop to retreat (and his knight to take over d5). On move 23, White's bishop is actually blocking his knight, making both pieces less than optimal -- which is why your move ...Nxd5 is a serious blunder because it solves both of his problems in one move.

@domprocopio I don't think a good player would take the dark squared bishop with the knight here. White has weak light squares around his king not dark squares (as long as he guards f2), so the dark squared bishop isn't as great an attacking piece as you'd think, esp compared to such a powerful knight. The knight controls a lot of key squares defensively and offensively, while the bishop needs some time to get operational. Meanwhile, White can play c4 if allowed (Black should still play it) and really clamp down on the light squares.

Yea, I remember this game, you played really good! You are a good opponent