Needing to add an outlet but confused why builder used 14-3 wire on a non-switched outlet
From your pictures, I don't see the third conductor.  Is there a red wire that we aren't seeing?  From what I can see it looks like he was daisy-chaining the outlets by creating a pigtail.  Since he didn't use the second set of terminals (or the first for that matter, not the way I would have done it) You can just pull power directly off the screw terminals.  Just remember to attach your ground.  If I'm missing something, please update your post and I will as well.