Because I am doing karate, it follows that I am also doing kata. It is one of the few things that all karate styles, schools and organisations have in common (daido juku notwithstanding). It doesn't seem to matter as to what use you have for them, but if you're not doing kata, you're not doing karate.
Beyond the definitional usefulness, why do kata? For most karateka, there will be several reasons, but usually one will predominate. I have considered these as stereotypical schools, but in reality, there will be elements of several in any karate organisation/dojo.
So, why do kata?
For some schools it is, in the immortal words of Tevier, "Tradition!". It was passed on to them by their sensei, or their sensei's sensei, or from the founder of the style himself. They do it because they do it. There doesn't need to be rhyme or reason to how or why kata is included beyond this.
For others, it is art. For these and the next category, kata are "performed". How it looks is of prime importance and the form of a kata gazumps its function every time. For many of these schools, there is no functionality. It's not needed, and gets in the way of a good performance.
The other performance schools are those for whom kata is a competitive endeavour. The kata's function is to be honed to a finely crafted piece of performance where each millimetre of positioning of the limbs and body is crucial. Why do certain movements in a certain way? Because that is the ideal that the kata is being judged against.
Some schools use kata as curricular material; thus on discussion fora you will see people discussing which kata they are learning for their next grading, or the scandalous occurence of a brown belt performing a (in hushed tones) "black belt kata!". Kata for these schools are a means of enforcing an hierarchy and creating a sense of "secret" knowledge, in that only certain levels have access to certain information. In order to gain this information, the karateka needs to toe the party line and rise through the ranks, gathering their knowledge piece by precious piece. The functionality in the kata is there as a discriminator. People who have learned their kata this way tend to get irritated or even outraged if others learn the kata out of turn, or "before their time".
For others, kata are a form of martially-oriented calisthenics. The stances are exaggerated to enhance their difficulty, and vigour and repetition are promoted as of prime importance. The techniques exist to be physically demanding of the practitioner.
Then there are the schools for whom the kata is a catalogue of martial techniques and principles. The kata contain the means to deal successfully with any attack. Diligent practice and analysis of the kata will provide the keys to these techniques.
Superficially identical, but in reality almost the opposite are the schools for whom kata are a solo form of practising techniques and principles. The difference is, is that the techniques are already being practised and honed in partner-training, and the kata hone the techniques and allow them to be practised alone. Analysis of the kata is not a major focus as the techniques and principles are known and already being trained regularly.
Which school(s) do you belong to? Are there any I have missed?