Cheeks are a part of the human body where the different pheromones are easiest to record. They telll information about the immune system, and hold the “personal smell” that everyone has (and which isn’t really a smell at all, because pheromones are observed with a different system than actual smells are). When you get your nose near someone’s cheek, you record a lot of personal data about them in your subconscious.

You receive information about how healthy your children would be likely to be, and also your subconscious gets irrefutable proof of “I am currently very close with person ______.”

Plus the other reasons mentioned here. But the most important one is that it’s a way to get your nose near their cheek.