Why does Rumpelstiltskin want your first-born?

This never made any sense to me. Why would Rumpelstiltskin want a child? What benefit does he get from that? If he needed a child for labor, or for adorable faces and cooing, or for baking into pies (because, sometimes that’s a thing in fairy tales), couldn’t he just get an orphan right away, instead of having to wait years for one woman to have a baby? Did the Germans (I think Rumpelstiltskin is a German tale?) think first-borns have some kind of magical power? Or that was a common thing to trade in their culture? Why not “give me 3 years of hard, manual labor” or “give me the map to El Dorado” or something that seemed to have an actual benefit to Rumpelstiltskin, not just a downside to whomever makes the deal? Or… What’s going on where Rumpelstiltskin says “Give me your first born”?