The most obvious reason why cats lick each other is to groom. Cats have a special tongue that is designed for grooming, and they use it to remove dirt and debris.
Licking is also a way for cats to bond with each other. When cats lick each other, they are exchanging pheromones, which helps to create a sense of trust and familiarity.
Cats also lick each other as a way to mark their territory. When a cat licks another cat, they are depositing their scent on the other cat's fur.
Licking can also be a way for cats to show affection. When a cat licks you, they are essentially saying that they love and trust you.
Licking can also be a way for cats to relieve stress. When a cat is feeling stressed, they may lick themselves or other cats as a way to self-soothe.
Cats with allergies may lick themselves excessively in an attempt to relieve the itchiness.
As you can see, there are many reasons why cats lick each other. It is a normal behavior that serves a variety of purposes.