Why play london system with c5 as black

So I've recently been studying the London system, as I'm new to the game, and it seems pretty standard, however, I'm stumped about why you would move c5 when facing the threat of a free pawn capture on d4. Is this just to break up the center, and move Nc6? why would one want to ever play this move?

Black playing c5 is actually a very testing response to the London.  It controls the center and establishes some hold on the queenside.  Taking on c5 doesn't give White a free pawn.  Black will play e6 and Bxc5.  White can try to protect the pawn, but it will soon be lost.

From Chess.com's database alone, you will see that Black is doing very well after 3...c5:  https://www.chess.com/explorer?moveList=d4+d5+Nf3+Nf6+Bf4+c5&ply=6

some lines to show that white can't really keep the pawn on c5. just my own analysis, unassisted. could be improved upon, but the general consensus is that white can't really keep the pawn, and if he can't then there's no point in surrendering the center with dxc5. it's equal, and white pretends to play some kind of queen's gambit accepted with colors reversed with a bishop on f4, but black has no problems.

Do you know Gambit?

This is the Queen's Gambit Accepted. with the same idea you as what your opponent plays

I don't play London, but I'm pretty sure if you try to hold on to the pawn bad things will happan