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.