I. Symbol of Status: When one cat grooms another, it signifies that the cat grooming another has a higher status, while the cat being groomed has a lower status. Generally speaking, cats that feel they have a higher status and occupy a leadership position will groom other cats to indicate that they are taking care of them and that the other cat will eventually be under their control. 
II. Marking Scents In the cat world, they identify each other by scent, and cats in the same family share a common aroma. Licking their fur effectively transfers this scent, so when an adult cat licks a kitten, the kitten's scent becomes identical to its own.
Third, grooming. One benefit of mutual grooming is that it helps clean areas that you wouldn't normally lick, such as your head, ears, or face. While you can use your paws to clean these areas, it's never as thorough as licking them yourself. Cats love to groom themselves, so remember to give them hairball remedy regularly.
