We lick our lips and cats lick their noses, we're not so different.
When I refer to a cat's way of scratching his head, I am not referring to an itch but a cat's version of the human action of scratching our heads when we are uncertain and thinking about something to which we can't find the answer and are a bit agitated.
I know when my cat is agitated and uncertain about my actions towards her. She is a nervous girl anyway. Actually she is a lady cat in her 80s (by human years). She has spent all he years a little fearful of everything due to a difficult start in life as a stray cat. This has made her less active and consequently a little overweight. Yes, I've tried to keep her weight down but she always wins the argument.
When a cat becomes a little agitated she will use her flexible and highly useful tongue to lick her nose. We scratch something and she licks. Actually we also lick our lips when we are talking and are concerned about what we are saying. That is why some people have slightly red or dry lips some of the time. They are probably more agitated than other people. There is then a similarity between a cat's instinctive behavior and ours which is to be expected as we are both animals (here I mean "animal" as a living creature on this planet).
The action brought on by uncertainty as to what to do next is typical of all animals. The action is a way of breaking out of the mental "log jam" created by being unsure about how to deal with something. Humans biting their nails is, I think, another example.
As for humans there are various versions of what the experts call "displacement activities". Activities that take you away from and give relief to the difficulty.
A cat might also yawn as an alternative. Licking the lips and the nose is the favorite though.
Photograph reproduced under creative commons copyright: danilou (Flickr)