Board Analysis - Pirc Defense - ~1600 ELO - Why does Stockfish recommend 12... c4 in this position?

While looking at Stockfish's analysis of a game, it found this position to be a critical turning point in which 12... c4 turned the tide of the game. 

Can someone help explain the tactics and/or positional play that would help find this move?

It is a positional reason. After c4 white can't take it, because b takes c4 and the white bishop is awful and white has problems on the b file. White can't go d4 because e4 is hanging. If white defends d3 with Qe2 then Rc8, then take on d3 and and Nc4. For the first sight...

https://www.chess.com/club/analysis-with-cm-u2ro/join/1fc964

Ok but even line I was looking at 12 ...c4 13pxp Nxc4 14BxN pxB 15Nb-d2 is only a nice plus fpr black, and it's about the third best line according to software. White isn't being knocked out of the park after 12...c4, especially at 1600 level.

Stockfish doesn't know, that a 1600 is playing :)

I love nice plus :) Now I see that after b×c black can take e4 too, but I didn't check the softwer. What does it say, which is the best?

12...c4 13.dxc4 Nxe4 doesn't look right. After 14.cxb5 white is threatening Bd5.

Software thinks something like 12...c4 13Nb-d2 pxp 14Qb1 and a later Qxd3 is best, with a small advantage for black.