Polydactylism occurs in cats because of a spontaneous genetic mutation. It just happened because the reproduction of animals through conception and birth is not perfect. Nature is not perfect. Copies of DNA are sometimes imperfect and these imperfect copies are mutations.
If a mutation results in a phenotype which is beneficial to the animal such that it survives more easily the mutation sticks around in perpetuity. This is evolution per Charles Darwin. It is the natural selection of the fittest per Darwin’s theory.
In the case of polydactyl cats the mutation causes cats to have extra toes. This is neither here nor there in terms of survival. It does not benefit survival and it does not detract from survival either. So the cats with extra toes did not fade away. Also they are liked by people who may have deliberately allowed them to breed.
The anomaly of poldactylism was first spotted in 1868 and perhaps earlier. It is thought that the gene that causes it is dominant and is symbolised by Pd. There is considerable variation from cat to cat in the number of toes and how perfectly formed they may be.
But the bottom line is that polydactyly in cats occurs because of a defect in nature which allows a genetic mutation to occur randomly. There are many examples of genetic mutations affecting cats’ anatomy and appearance. Well known ones are the hairless cats: Sphynx and Don Sphynx. Another is the folded ears cat, the Scottish Fold and another is the tailless cat: the Manx. There are more. The hairless cats would not have survived normally because a lack of hair is severely detrimental to survival. They survive because humans want them to survive.
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