Ummm... because you're using a tablet computer. You NEED to keep it on for your alarms to work. Or do you expect your bedside clock to wake you up if you unplug it? And no, iPads cannot turn themselves on for an alarm, only if they're on Standby (which is what you'd need to do with your tablet if you want it to sound the alarm).
And as long as you're not actively using the tablet, having it on (on standby mode, when the screen is off) shouldn't be that much of a drain. Yup, iPads are way better at conserving battery at standby than Android tablets, I'll give them that, but still...