Steel plates are not inserted in elastomeric bearings, but they are embedded inside the rubber at the time of manufacture itself so as to assure that the plates are completely embedded having protection against corrosion, which could be a problem if they were inserted later. They are generally vulcanized or molded into the bearings like a multilayer sandwich, although the manufacturer may also connect the plates to the rubber via additional steel connectors.
The primary purpose of steel plates is to act as a reinforcement for the bearings against
1) vertical compression,
2) vertical compression with horizontal shearing,
3) vertical compression with horizontal rotation. They provide for higher resistances in the bearings by allowing limited horizontal deformations without failure at higher loads than the rubber itself.
Sometimes, steel plates are also used externally to provide resistance against high horizontal loads, but these plates aren't completely or sometimes even partially embedded.