I don't know very much about the history of kata, but anyway.
Kata = Form
It was started several years ago by martial art masters, why? The masters did start to learn about "Tension" (Force), to make there more powerful they created kata/sanchin.
Kata/Sanchin is all about tension/form, the more tension you can generate/create the more power you get into your punch/kick.
It's not only about "tension" it has another purpose too.
the second point of the kata is to learn the techniques that are in the kata, so that we could use them in a necessary situation.