Here's a short post to finally answer (at least give the best answer I can give) about the mysterious JPs from GTA Online. First of all, just in case anyone didn't get that yet, JP stands for Job Points (not Josh Points, or Japan Points, or whatever). There's 2 simple reasons behind the existence of JPs: To show other players if you play a lot of jobs, and to compare the different positions in different kind of jobs (and obviously they are used to make the leaderboards in playlists, and as tiebreakers on the next job voting screen).
Here's the official answer from Rockstar themselves, from 2013 (and yes there's a huge watermark on it, because you won't find that anywhere else. I also blurred a few parts, out of respect for Rockstar).