While people tend to think that cat's groom themselves, this isn't strictly true. Cats lick themselves, and this can be the cause of hairballs, so most longhaired cats would benefit from grooming.
They are given a bath because their coats can be greasy, before being dried and combed out, this will also help with the amount of hair being shed thereby reducing the hairballs the cat 'throws' up. Also when people say they are allergic to cats, it might only be they are allergic to the dander of cats, so getting the cat on a regular grooming schedule can reduce the amount of dander and help with the allergy.
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