Here's what I understand about the point of c5 in the French / Advance Caro-Kann pawn structures: if White takes you are very happy, because his e-pawn becomes weak and you get to develop your bishop to a nice active square in one move. Even if he doesn't take, it opens up some breathing room for your pieces, eg. you can play moves like Nc6 and Qb6 to further pressure the d-pawn, eventually play cxd4 to fix the weak pawn on d4 and open up the c-file, etc.
However, I don't understand why it's as desirable in Scandi / Caro-Kann pawn structures where Black has played dxe4. In these structures:
- White has not committed a pawn to e5, so you are not making it weak.
- In most cases you've already developed your bishop to e7 or d6, so it's not like you will be saving a tempo if he captures.
- Also in most cases, you have already played Nd7 so you're not gaining the c6 square for your knight.
- You've already placed c6, so your queen can already access the a5-d8 diagonal.
- If you capture on d4, White can just recapture with a piece without having to worry about the pawn on e5 becoming weak.
So, in concrete terms, what does Black gain from playing c5 in these structures? (And ditto for e5.)