1. Their heads are too heavy. Some cats seem to be chronically lazy, and they will lie down if they can. They may feel that their heads are very heavy, so they need to find something to lean on to relieve their pressure. 
2. Stay alert. Cats are used to keeping abreast of their surroundings. The chin-resting position allows them to keep their heads at a low level, making it easy for them to check their surroundings.
3. To assert dominance: Cats have many glands on their heads that secrete unique pheromones to help them mark their territory, which is why cats like to rub and scratch their heads everywhere.
4. It's simply a matter of preference. Cats like to rest their chins on their hands, just like we like to prop our chins up with one hand—it's just a habit.
