Kyuubey created Mami and uses her to make more Magical girls

Basically when Mami and Kyuubey are first introduced, Mami is guarding Kyuubey
 from Homura in the sense that she consistently is around her, or in some way, shape,
 or form is around her constantly. Evidently there are many magical girls created
 through wishes granted by Kyuubey, yet they are no where to be found, we are to
 assume that Mami and Homura aren't the only magical girls in existence right
 now, and as such Mami being around right when Kyuubey needed it was all too convenient.
 Which brings up the second point of how when Mami died, Kyuubey immediately beckoned
 the girls to contract with him, without so much as batting an eye(like he does
 anything else), which brings forth the idea that Mami, or a variant of her, is used
 each time in the process of creating new magical girls. It first gives potential
 magical girls someone to look up too, relate with, and teach them the dangers of
 what it really meant to be a magical girl, while still making it something that
 would be appealing in the long run. Eventually reaching the traumatic death 
of "Mami" in any of her forms, forcing the hand of the girls who may be wavering to
 quickly become Magical girls to save their own lives, or influence them to the point
 of eventually becoming magical girls in the near future. 

外人の仮説を読んでいるよ(´ー｀)