Many people/masters/schools have different views on what Katas is and is for. Below would be my views on Kata
What KATA is.
It is basically a set of moves a practitioners could do on their own, as well as with another partner.
What KATA is for.
This question is very subjective depending on the school / master. From my point of view, KATA in its essence teaches proper structure,form, footwork, proper utilization of power and leverage and etc. By perfecting your KATA, you could generate the maximum effectiveness of your art.
For example, i feel that my punches tend to be weak and tend to leave me unstable after delivering a solid blow. By perfecting the KATA, i am able to understand the principles and reason why it is weak, thus allowing me to make corrections, and after many many times of repetition, it overwrites my previous form or bad habits.
There are also many hidden moves & principles in a KATA and it is usually difficult to identify unless pointed out by someone else.
Many practitioners find that by perfecting KATAs alone would make them a complete fighter. They often neglect the physical conditioning part of training. By lifting weights to make you stronger & faster, sprinting to increase speed & stamina, meditation to increase focus and etc. These are all as important as KATAs and should be trained equally.
In the beginning , you might be able to get away with sloppy stance, sloppy punches or kicks just by being strong or fast, but as you progress and meet more experienced practitioners who are as strong and as fast as you are, you wouldn't be able to rely on physicality alone.