I think the reason why Spamton used this item for his sweepstakes is because Jack Skeleton and Sally are representing Papyrus and Dess.
The people he considers as the King and Queen of Deltarune.
He either wanted to be Papyrus for having the greatest freedom, or Dess because was a broken girl put back together and finding acceptance under the Papyrus.

The knight is the minion of Papyrus, assigned to Papyrus by W.D. Gaster.
That’s why she has the ribcage and the holes on her hands. She’s Papyrus’ skeleton knight. And Susie is meant to be Papyrus’ dragon. Hence this snow globe.