From W2AEW:
Published on Aug 9, 2014
Shielding is used on RF circuits for many reason.
The most obvious is to prevent the circuit from radiating RF and causing interference in other circuits, and to prevent RF signals from causing interference in the circuit of interest.
Another important reason is to minimize the parasitic effects of surrounding objects - this latter point is illustrated clearly in this video using a varactor controlled VCO (voltage controlled oscillator) and a shortwave receiver.