Why are steel plates inserted inside the bearings in Elastomeric Bearings?

because it is responsible for a vertical stiffening effect and the simultaneous reduction in the rubber swelling.

Please visit the following link to read more about the bridge bearing:

https://theconstructor.org/structures/bridge-bearings-types-details/18062/

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.

Here, I am discussing some important reasons behind Steel plates inserted inside the bearing, especially in elastomeric bearings are as follows;

1. For restricting the freedom of bearings to bulge and hence the deflection will get reduced when compared with bearings without any steel plates under the same load.
    
2. Inducing tensile stresses which limits bulging of the elastomer which provides a limit to the thickness of the steel plates
    
3. Steel plates have the special property that it doesn’t affect the shear stiffness of Elasticmetric bearings.
    
4. For increasing compression stiffness of elastomeric bearing by limiting the amount of lateral bulging
    
5. To function the bearing as a soft Spring in elastomeric bearing
    
6. Bearing should be allowed for bulging laterally in elastomeric bearing (Above diagram can clear your some doubts so refer it)