the earth wire is used to protect the user from any accidents that occurs using a metal applications.if there is any problem in the application the case itself becomes live with the possibility of electrocuting anyone who then touches the case. discharge any stray charge on the body,if earthing is not done then it will shock if touched.Without the earth wire, you can get a shock. an
Earthed wire, permanently connected to the case will provide an immediate path for current
to safely flow through .. so the case remains safe to touch.