I'm not as obsessive about my online calendars all the time as I wish I were — I would never forget about the occasional appointment or date I made on the fly. Even though I use calendars, sometimes you just need really straightforward reminders — which is what Deadline is great for.
For instance, I have a doctor appointment on my calendar, but what I really need is an alert a half hour before I need to leave for that appointment. I just need to add a deadline from the home screen, set the time and date, and make sure I set when I want to be notified by email so that I can get to my appointment on time (I set it to go off a half hour before). No more sheepish calls about why I'm not where I was supposed to be!