My old cat George used to alternate between snoring (like a cement mixer) and purring (like a revving Juggernaut) all night. I didn't sleep well for years.
I thought they purred because they were feeling safe and confident and ready to be stroked. I've had a cat who purred even when she in pain (they go awfully quiet though), so I guess it was because she was willing to be helped out (she had a nasty abscess and we were waiting to see the vet).
I know what you mean wolf - BigThug sounds like a woodpecker with a cold when she starts !
I think they purr when they feel safe and are having a lot of pleasure from stroking, combing....I have also heard, however, that cats sometimes purr when under great stress.
I doubt Jessie is under great stress when buried under my duvet, pawing my face and licking my eyes though! lol
If a cat wants to sleep in your bed it WILL!!!!
My Whirly is a Bengal so as well as an enormous purr she she's got a huge range of loud noises (and no volume control or mute button). She can purr and eat at the same time too.
Boots is a felix lookalike and regularly purrs himself to sleep.