My cat does this too. She watches me turn on the sink, then she paws it, after, shakes water off her paw. I think it’s curiousity. And maybe it’s cooler, or maybe she just likes it. Or she can be marking her territory. Cats roll around to put their scent there so they can come home if they go out.
Because its like a safe hideout. Also, its a cool place to lay down, like being in front of a fan for you. high up to pounce on prey. Is your kitty getting pleanty to drink? If you have a drip he may lay there to get some water.
Cats like a nice, secure place to sleep. Many like to sleep in close quarters because it feels safe and secure to them. My cat loves to sleep in the bathroom sink but he is getting too big for it! It can be amusing to watch him try to squeeze in there to attempt to become comfortable.
You will also notice that when you bring home new shoes and the cat hops in the box and falls asleep? Same reason, cats seem to like close quarters when they sleep. A friend of mine has this huge purebred Ragdoll, that will do this, and the box will burst at the seams and will be close to falling apart, but this cat is in heaven and will sleep the day away in that box!
My cat does this too. He seems to like the small space that is cool to curl up in. The small space may feel like a den to her and she might like it more than her bed because it’s familiar to her. Some cat breeds actually like water(my cat likes water and will sometimes swim).