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. The VCO is the same circuit that was used in the Varactor Diode Tutorial video: