I read that it is asking permission to hunt in your territory.
Someone else suggested it is to mark you as their own (territory?)
Which is it – asking permission or granting permission?
(Okay, a cat, should I really ask this?)
Cats nuzzle lots of things. They have sebaceous glands in their chins which leave scent. My cat books all lean toward your second guess.
They are marking you as theirs. Being owned by 9 cats, I guess I smell really strange to other cats. “Mine” No, mine!" No, mine mine mine!"
I’m leaning on this explanation. 
I feed a feral and the only time she rubs against me let alone get close is when I’m putting food in the dish and she has to eat fast or Tom’ll get it.
It’s kitty wuv. Just like when they knead you and leave runs in your clothes and/or welts in your skin.
It’s just trying to say, “We’re pals, you and me, so let’s keep the peace.”
As with so many millions of other behaviors of animal and man, the answer is:
“Because it feels good.”
The next logical question, then, is “But why does it feel good?” For that, I’m afraid you’ll have to check in with those brave scientists working to solve the baffling mysteries of warmblooded interspecies affection and random spot itching.
Marking territory works. Except for the fact that I almost never see my alpha cat rubbing up on things - it’s only my beta cat. :dubious:
I prefer to avoid this guesstimate and go for the “They want to jazz up the clothes you just spent several minutes de-linting”. 
Any theory that relies on a cat asking permission for anything is inherently faulty. So I would toss that one right out.
The evil bastards are trying to trip and kill you so they can devour your flesh.
I don’t think the scent-marking explanation is complete. Cats (and other animals) often communicate using behavior that has some other primary purpose. For example, they will sharpen their claws to express playfulness. The primary purpose of sharpening the claws is to, well, keep the claws sharp. But when a cat looks at its owner with dilated pupils while hooking the scratching post, it has more in mind than just keeping its claws in shape.
In my experience cats usually rub against an owner’s legs when the person comes home after being away for a while. I believe it’s a form of greeting. Some cats use it to mean, “I’m hungry, feed me.”
I call one of my cats “rubber monkey” because he’s always rubbing his face on everything. One idle morning, I watched him rub the side of his face on my bedpost for at least three minutes straight. It was kind of freaky.
I think it’s obvious that they’re they spreading their scent around, but I also think that it feels good. The same areas that they use to rub (the side of their face and the top of their head) correspond to the areas they seem to like being stroked and caressed.
Cats rub the sides against you to get your attention. They want something from you and are making their presence known.
They their chin to mark things.
my cat is frantically rubbing his face against the side of my monitor and leaving hair all over the screen, as I type this he is rubbing my hands, the cords behind my computer my coffee cup(gross)now my hands again, the side of my desk…he is laying
across the key board…he acts like he is just trying to get my attention with the rubbing thing. He gets really needy when I’m at the computer.
It wants your attention. It means scratch me now and then feed me!