I use the alarm feature in the iPhone's Clock app to remember when to feed my cat. I find that she bugs me less if she thinks the alarm, and not me, is in charge of the meal time (it sounds stupid but it works). When I'm out of the house, though, this alarm is mostly just a nuisance. That's why I set up my alarms to only trigger if I'm at home.
This is useful for more than just cat people. Maybe you don't want your usual morning alarm to trigger while you're on vacation, for example, or...