With a large enough library of music, I've found that it's simply not possible to mentally keep track of everything I like listening to. Simply put, there are huge swaths of tracks that are unlistened-to-recently. In the absence of time to better organize my iTunes library, I resort to using the shuffle feature. Just point iTunes at my library, hit the little X button and Play. If a track isn't appealing (or it's an iTrip station), I just hit next, like a reflex.
Pro: Lots of library coverage, and I'll end up listening to things I wouldn't otherwise remember I had. Con: No continuity. However sophisticated the shuffling algorithm ends up being, it's not the same (not the same being equivalent here to never gonna be anywhere near) as a back-to-back listen of a well-produced album or mix set.
Music helps me concentrate. Once I get going down some mental path on task X, the continuity of a record helps keep me on track. A library-wide shuffle doesn't. My mind might settle into a pattern after a few minutes, but by then most tracks are over. Then it's a subtle shift (depending on the set of music being shuffled) to another record, and a mental reset.
Navel gazing moment: Found the inexplicable appeal of minimal repetitive electronic music? Bass tastes good.