Had a cat (female) who slept on my back for 18 yrs straight every night (just about, give or take). It probably started out as 'possession', but maybe also keeping warm (she was a glutton) and then just habit. Luckily my husband wasn't a flailer in his sleep!
I would'nt think of it other then snuggling unless there is an aggresive reaction when you move. Some cats love to snuggle up when we are sleeping! My friends chawawa sleeps right on your tail bone ever since he was a pup
Shami likes to snuggle right up in my face and licks my face or kneads my neck. After about 15 minutes of me turning about and him following my face, he'll give up and curl up on my back and fall asleep, or I'll grab him and lay and arm on him so he doesn't leave claw marks all over my neck or tongue rashes on my face.