Yes, cats kneed to pump mom's milk. Now, why do they purr? I have read that mom's purr is there to "call" the newborns towards her to eat. The kittens then learn to purr back. Purring and kneeding would both have to do with nursing and bonding.
Last I heard, no one has yet figured out HOW cats purr-physiologically.
Let me know if I'm wrong but from what I understand, scientists have been unable to explain the how (maybe the where? but not the How?)