Eggplants are a sturdy vertical plant and don't normally need to be caged. If you get lucky and you grow heavy eggplants, it wouldn't be a bad idea to keep them off the ground with stakes. I have never had to do that.
Squash is a horizontal growing plant and can sometimes get heavy, and leaving them on the ground can lead to discoloration on the ground side. We have all seen the light spot on watermelons. Putting a barrier under each individual Squash is what most people do. I have seen up-side down Pie Tins under Cantalopes, Melons and Squashes in local gardens to keep them off the ground.
I have heard of People putting their prize winning Giant Pumpkins on a Pallet when they are small, but that might be just to make sure they can lift it up easier when it is full grown. In some cases, having a barrier under something heavy can help.
You shouldn't need it with Eggplants.
