Various reasons. Love the show, love the actors and want to see/support them and their show, love Green Day and by extension the show, or 'cuz they want to blow their hard-earned discretionary $$ on seeing the show several times instead of doing something else with it that's less important to them.
I saw the show for free during opening week last April as a result of somehow winning an MTV contest. My wife and I are going back to see it in January (3pm matinee on the 9th) and then go to the evening panel interview with bj and Michael Mayer. She hasn't seen the show yet but thinks going up to NYC for the interview is a great excuse to take care of that little problem. As for me, even though I've seen it once already, one thing I came away from after seeing it the first time is that there are tons of subtleties, sub-plots, etc. woven into it. It is a very, very rich show as a result of how much further interpretation of Green Day's album that Billie, Michael, Tom (Kitt), the actors themselves, and the rest of the show's creative team has added to it. There is something going off in literally every corner of the stage throughout the show, including on the monitors, and I doubt there is any humanly-possible way for anyone to see it once and not miss something while they're there. It's like any good movie, good literature, or a good album -- people who really like it the first time and can see that richness want to see/read/listen to it over and over again in order to glean everything they can from it. In my opinion, AIOB is one of those shows that's worthy of this kind of focus.