Why do cats purr?

I was motivated to find out why Morphy purrs and perhaps what makes him purr more than most cats. I assumed that science would have an answer to this question. However, after extensive research (i.e. I looked in Wikipedia and one other website) it seems that nobody really knows for sure!

Apparently it's not just an expression of contentment (which would seem evolutionarily odd anyway). The most plausible explanation I see is that it is used as a signal from the mother to her kittens, the kittens being able to feel the vibrations of the purr more easily than discerning any other vocalisations. In adulthood, it is more an expression of friendliness or good will, in a similar way to a person's smile or laugh, to indicate absence of threat. I can understand such involuntary expressions of emotion in a social species, but in more independent animals like cats it is more surprising; hence it seems it must be a remnant of a childhood trait. Perhaps purring has been selected for in domestic cats, making its evolutionary purpose, genuinely, to please people! But be wary of that theory - wild felines purr as well, even cheetahs and tigers!