If we ask a students to do whatever they want to do, they do whatever they already have learned in the past. The amount of creative thinking may be zero. When there are limits, there is a better chance of having a challenging task. The teacher's challenge is to make the limits seem compelling and interesting to the student. We Kill Creativity if we Give an Answer instead of teaching Problem Solving methods.
Artists use a variety of problem solving strategies.
Some know that they need to simplify. Some need to work at creating new kinds of order from chaos. Some want to point out the problems of the world. Some want to search for more perfect beauty. There are many methods of working aesthetically, and art teachers need to expose students to as many as feasible.