Question

Why are electrical appliances with metallic bodies connected to the mains through a three-pin plug, whereas an electric bulb is connected with a two-pin plug?

Answer 1

Electrical appliances with metallic bodies are connected through a three-pin plug because the earth wire provides safety by carrying leakage current to the ground and prevents electric shock. 

An electric bulb uses a two-pin plug because it has no exposed metallic body, so earthing is not required.