The whole Pitch Class set concept was a big deal in the early 1900's. It evolved into 12-Tone "Tone Rows" which is a form of serialism. It's not a terribly popular way to go about composing any more but the guy who wrote the film score for The Matrix movies used it as a way to organize themes based on subject material.

I "get it"...what all the terms mean, but I can't say that I'm enthusiastic about it. It's important to know from a historical standpoint, but tonight I'm supposed to analyze a serial 12 tone piece by Ernst Krenek and I'm staring at this thing and really...I couldn't give a shit. His stuff is almost as bad as what I just put up here, my little 45 second piano piece.