Because of spontaneity. Some days, I could wake up and decide, “Hey, let’s go to Disney today,” or finish work early for the day and be like, “Wanna go to Disney tonight for a few hours and ride a few things and get ice cream?” That’s something that could be done with APs: Being able to go if and when the mood strikes (so long as it’s not a blackout day, obviously).
Now? I personally don’t want to have to plan every single time I want to go to Disney days, weeks, or months in advance. Planning ahead because, say, a friend is visiting from out of state on a given day is one thing, but reservations kill the possibility and flexibility of just being able to up and go on a whim today because of having an unexpected day free or something.