Cat grooming cat. Just like cat grooming human
Millions of people have asked and answered this question. There are hundreds of websites answering it! There are a dozen new ones each day. I will throw my two pennies worth into the ring. I think the answer is obvious.
Mothers lick their newborn kittens to wash them and when older to groom them. Siblings will groom each other. Unrelated cats that are friendly to each other, even very close to each other, will also groom each other.
Grooming another cat serves the function of cleaning the other cat and keeping the coat in good condition and it is a pleasant experience for the receiving cat. You can see the look on the faces of cats being groomed.
Mutual grooming also serves the function of bonding the cats. This is bound to be the case. Because it is pleasant for the receiving cat, he or she is bound to want some more and approach and be close to the grooming cat - and vice-versa.
So when adult cats groom each other it serves two purposes as described.
People who care for cats are seen as mother cats by their cats. This is because the person provides for the cat in all major aspects of life in a similar way that a true mother cat would for her offspring.
The difference with the person/cat relationship is that the cat never grows up because the person provides for the cat all of his or her life.
Bearing in mind therefore that the human/cat relationship is a cat to cat relationship from the cat's point of view, when our cat licks us he is licking a cat in an act of mutual grooming and bonding.
In response we do not lick our cat. Shame really. We have a poor substitute. We stroke our cat. This serves the function of bonding but not maintaining our cat's coat.
Gentle flea combing is like stroking and also serves to maintain the coat and the cat. Gentle and routine flea combing is probably the nearest we can get to licking our cat in return for her lick on us.