Kata help reinforce the techniques, teach discipline (especially for the kids who want to rush through it), help strengthen our legs and reinforce the stances, and help with memory and neurological pathways (we had a brain cancer survivor who found the forms to really help her get her mind back on track).
My reasoning is as I said above. In middle school, I was practicing my basic arithmetic in the context of algebra, i.e. if I needed to do 4x + 5x = Y, then I'd still need to know 4 + 5 = 9, so 4x + 5x = 9x. If the advanced form includes all concepts covered in previous forms, but expanded on, why do you need to practice the previous form?
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It sounds to me like your kata are structured differently from the forms I train.
We have certain fundamentals that are consistent to our method and will be found in all of our forms. So those things are constantly trained and given work, regardless of what form we are doing.
But otherwise the forms are rather different in pattern and applications involved for the techniques. So learning one does not replace a previous one. Rather, you are increasing your repertoire.
Forms are a tool meant to help develop certain skills. I donāt like to speak in terms of being good at a form because to me, that implies the form is a product meant to be performed and I do not agree with that. Rather, a form is just a tool we continuously practice in an effort to improve our skills. In that way it isnāt any different from hitting a heavy bag: you just keep doing it in an effort to improve. You are neither good at, nor bad at, the form, but a good teacher can gauge your skill by watching your form. Likewise, getting good at hitting a heavy bag is not the point, but hitting the heavy bag helps you get good at hitting an enemy. And a good teacher can judge your skill to some degree by watching you hit the bag.
Theoretically, once you really learn and master a skill, you could make an argument that you no longer need the tool and therefor discard the form. But in reality I believe that the form continues to be a useful tool for practice, forever.
Honestly, I believe that some systems pile up a list of forms/kata in their curriculum that can grow too large. When that happens the curriculum becomes cumbersome and difficult to practice because there is simply too much to do and not enough time. A form needs to be practiced frequently in order to get benefit from it. If you have so much material that you only do each form once a month, I doubt you are getting much benefit from it. I think it is possible to get a lot of mileage out of a modest amount of material, and I am a fan of that approach, rather than building a curriculum that is so large as to be cumbersome. I feel that three to eight forms, well understood and trained regularly, can be useful, and if you have more than that, it becomes questionable. But that really is for each to ultimately determine for themselves.
However that may be, I feel that if the material is codified as part of the formal curriculum, then you donāt throw it away after you learn something else. If you begin to feel that the curriculum is too large then it might be a sign that you want to consider a different system. If you are a highly experienced person you might consider consolidating what you do and streamlining your own personal curriculum, which could mean ejecting some material if you honestly feel it isnāt necessary. But that is a personal choice and might mean you are on your own and no longer training with your school.