Queen's pawn opening - Black at move 4, best move cxd4...why?

I get this position a lot playing Black in the Queen's pawn opening

Apparently cxd4 is the best move (I played Nc6, developing a piece as you're 'supposed' to do).

Can someone better than me please explain why cxd4 is the best move?

pressuring the knight to take tempo.

force them on the backfoot with their positional development.

center control, thought it's dicey to say "center" here. It's more side control imo;; if you do not take, your d4 pawn is taken by c5, now guarded by knight, and your position crumbles completely after you recapture that pawn.

taking the pawn and checking the knight and forcing it to move elsewhere, allows you to immediately capture his c5 with your d4 instead of the other way around, now you still have your frontline pawns AND your position expands by one rank on the board. Furthermore, d5 is now protected by the queen. From there you have many better options than black, vs if you had not taken D5.
Idk if that explains it. Is that your game?

(PM read, thanks!) Yes, my game and I am playing Black here - to be sure, the move in question is 4...cxd4 rather than what I played which was 4...Nc6.

#3 knows what he's talking abt even though he's 300. Also this opening is now the semi tarrasch. Not queens gambit territory any more.

Or is it the Tarrasch? I'm losing my d4 memory :(

After 4…Nc6 5.cxd5 Nxd5 6.e4 Nf6 7.d5 white got great center an initiative. Engine consider it +1