Dogs will usually rub their face on you as a way to display ownership and territory of their owner.
Dogs have scent glands in their facial area so they're able to mark you as their owner and then other dogs can smell the dogs scent on your skin from the dog rubbing it's face on you.
Sometimes dogs will also rub their face on your skin because the dog could also have itchy skin and is hoping that you will take care of the itchy skin and scratch the dogs itch.
It can also be because the dog is showing affection towards you and wants attention as well.