You're using a corded drill with a metal case. What's the importance of the ground plug? 

a) If there are limited cord options, the ground plug can be removed, so you can use a different cord. 

b) If the drill has a battery option, it can be converted to a battery pack using the ground plug.

c) If the power is lost, the ground plug ensures that the tool will still function. 

d) If there's voltage on the tool, the current will flow through the ground plug instead of through the electrician.