"Good breeding stock" is too good to leave to mere half-bloods, but by the Dark Lord’s rules, I don’t have the right to marry a pureblood. The solution was obvious: I was to be a concubine. I and others like me (and men and even half-bloods could get the "good breeding stock" label, too) were to be auctioned off to the highest bidder. Any pureblood who wanted me and could pay could have me and could do pretty much anything he wanted with me, including loan me out to his pureblood friends, as long as it didn’t hurt my breeding potential. (Thankfully, the Death Eaters were smart enough to know that most forms of torture would hurt my breeding potential.) It was a better deal than most other mudbloods got, even if I was still chattel, but it was a great deal for the lucky pureblood who bought me. He would have pureblood heirs through his wife, and in the meantime, he could have his way with me whenever he wanted, plus a few extra kids from me. In return, I’d be treated better than my peers, and my bloodline would be bred up to snuff in two generations, which was about the best I could hope for under the new regime.