Why do most elastomers used in pot bearing are usually contained?

To specify space requirements, most pot bearings are designed for high contact pressures with small contact area with bridges. This also enhances lower friction values. Under the free state, most elastomers in pot bearings can hardly sustain this high pressure and hence they are most contained to prevent overstraining. When properly constrained, the elastomer behaves like semi-viscous fluid and can safely accommodate angular displacement.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.