I know this is a scientifical question, as nowone excatly knows why… but it is the cutiest thing a cat has, so here is a dedicated post to them. Let’s hear about it (and I hope no one will talk about it being for the sake of their bone structure… it is such a lame answer…)
because they are bored and they love the sound of their own voice, …err own purr…?
because they can’t say, “oh ya, thats it, thats the spot, keeeeeppppp scratching” That and they sleep a whole bunch, an animal that is that lazy can’t be bothered to learn another language.
…because they don’t know the words…
Actually, cat purrs remain one of the mysteries of our universe. Seriously.
Researchers are not sure how cats even make this sound, although they do have theories about vibrating cartilege etc.
What I do know is that when my 2 cats purr, it makes my day. And they are surely the world’s LOUDEST purrers.
I think it is because they cannot say the “mmmmmmmmmmmm”, yes yes yes… we have to test if they do purr after a shower with Herbal Essences…
Because Cats are such a purrrrrfect pet…
Mine purr because they love me. You know, 914 said once in a PM that she thinks she “sometimes loves them (her cats) too much.” Well, she’s a girly man so it’s kind of forgivable, but I’m a MANLY man, and dammit, I found myself thinking the same thing last night.
Cats are uber-cool.
And they can purr while inhaling and exhaling. Cats rock. 
i also read somewhere that cats purr when they are in pain also. :s Now that is confusing, how do you know when it’s enjoying something or not. I know my cat purrs when i have to de-pee and de-poo it and i’m sure she doesn’t enjoy it. 
My cat also comes and sits on my pillow and purrs when she wants me to get up.
I have read that science studies have shown that the vibration or soundwaves or something(sorry can’t remember which) that a cat makes while purring actually help accelerate the healing process. That might explain why they purr when they are hurt.