At it's heart, a form is intended to build muscle memory. If you repeat any movement often enough the movement will become automatic. So if your movement is "textbook" the automatic expression of that movement will be close to "textbook" also.
The problem with forms is that when they are done without thought about their purpose and you don't vary the form so that the movements are done in different order you risk training your body to only respond in a limited way. There are plenty of instances where martial arts students loose a fight because the fought a form instead of an oponent.