It allows the witches to possess power beyond their limits. That common person who kneeled down in shock because of the miracle is the best proof. 

Chapter 688, related to Magic power

It mimicked the workings of a Magic Cyclone, trying to ask for power from the deities directly. In some sense, it could be considered as an extremely powerful man-made Magic Stone, way stronger than the Sigils, simple and crude toys as compared to a magic core, which could only increase a witch's power. However, experiment results showed that deities would never give a Key to a lifeless thing. Only a witch who had a matching Key could activate this thing.

"You can take the core as an artificial Magic Cyclone. It's not confined to a witch's body and can adjust its forms at any time. This bizarre device almost overturned the Union's knowledge system." Using one of her tentacles to point at the light curtain, Pasha said, "For example, the phantom instrument we're using for our communication is a minor scaled magic core. When necessary, it can be converted into other instruments. As to the Instrument of Divine Retribution, it's the biggest magic core in the ruin."