Madoka Magica: Just Sign On The Dotted Line

[Jorlem]

I believe it was stated somewhere that the Familiars are split off of their parent Witch, and can grow into a clone of that Witch.  Its kind of like how you can take a cutting of a plant, and grow it into a full plant.
I see Kyubei as a sort of catalyst, promoting the growth of the Grief Seeds.  That Mami called the Grief Seeds 'Witch eggs', and Kyubei's full name being what it is, seems to indicate to me that Kyubei isn't actually eating the Grief Seeds, but is in fact storing and stockpiling them for something.  I mean, look what happens

Spoiler

when you put eggs into an incubator.

-----
Stand between the Silver Crystal and the Golden Sea.
"Youngsters these days just have no appreciation for the magnificence of the legendary cucumber."  --Krityan Elder, Tales of Vesperia.