To foster some kind of competition, perhaps. To groom individuals from different worlds in the training of these creatures. Or to test the reactions against an environment as tilted towards combat as this one. Assuming this place is still serving its intended function at all.

[Or it really is just an elaborate freak accident of rifts in time and space, but Missy's not one to believe in accidents.]

Virtual or real, there's always an exit, for those capable of finding it and willing to use it. The apparently random nature of arrivals and departures just makes it rather more difficult than it might be.