I think the main purpose of forms (kata) isis muscle memory,memory; drill something enough, and it you will be able to do it without thinking.
theThe precision and exactness of the forms also encourages the participant to focus on the little details of a technique which will make their over alloverall technique much better.
theThe final reason iI see for doing forms/kata is discipline, doing the same thing over and over gets boring and it builds discipline and character to be-able able to get past that and maintain focus.
oneOne other thing some people may find that doing forms/kata is a form of meditation.