When you create a trust, it is important to think about the unexpected -- about the things that may happen that you cannot predict. You do not want to accidentally keep money from your heirs when you would have preferred to pass it on to them. You can't change the trust after you pass away, so you must plan for all possible outcomes in advance.
For instance, say you create an educational trust for a child that will just pay for school-related expenses like books, tuition, room and board and other such costs. However, your child gets married while in school, and then their spouse gets sick. They quit school to take care of them. Do you really want to say they cannot have money they may desperately need just because they're not paying tuition with it?