From Scientific American:
http://www.scientific...m?id=why-do-cats-purr
From me:
The cat sleeping alongside me on my bed purrs to tell me it's so contented that it would be totally unreasonable of me to expect it to move, so that I can avoid cramp in my legs.
The cat on my desk purrs to get me to accept that I should ignore the fact that it's just walked over my keyboard and deleted the important document I was typing.
The cat by the feeding bowl purrs to let me know how much it likes the expensive food I've just given it (even though the same cat will turn its nose up at the same food a few days later).
And the aforementioned cat purrs to make me forget that it's not even mine, because it's meant to live two doors down the road!
Chris