I find it difficult to fully enjoy musical theatre songs if I don't know the storyline of the show they are from as well as the context.
Of course I can appreciate the musicality of the pieces and sometimes the lyrics are generic enough to be listened to without reference, but I often feel cheated if I don't know what the hell is making this person burst out into song.
Frankly I don't listen to lyrics (a problem in that I apparently work in musical theatre) I just want a good tune that doesn't require the use of too much grey matter.
I can memorize great swathes of dialogue!