Forms are a method of Shadow Boxing in many styles. Ways to practice the movements and techniques without a partner. The more important aspect of forms however, is the transmission of an individual teacher's fighting system/methods.
Hailing from a time before YouTube, DVD's, VHS, or even the Printing Press, being able to pass down information from one generation to the next, is complicated at best. Forms are, at their heart, an ingenious way of passing on information without the need for literacy.