Every Tuesday, I start my work day at 8 AM. Other days, I start my work day at 9 AM. That means I need to wake up an hour earlier on Tuesdays. iPhone alarms do at least allow you to repeat alarms for specific weekdays, and can be multiple weekdays (and not all of them). That's all fair and good.
I don't really care so much about waking up on time on holidays and vacation days though. Or I would like to wake up at a different time altogether on those days, and I know well in advance what those exceptions are going to be. So I'd like to schedule them in advance, and override any repeating alarm I have.