See, XR at least the potential to last throughout the day. This is true for for many long/slow/extended release drugs.

Whether it is for the purpose of mood stabilizing, or say medication for ADHD; if you take a stimulant to focus during classes, you -may- find that your focus begins wavering when studying or doing homework later in the day.
Some people who take a immediate release of a sleeping tablet (especially those that are geared towards helping you fall asleep) will wake up in the early hours of the morning. The extended release has the ability to help insomnia of falling asleep and that of staying asleep.