Question

A green coloured wire is seen in electric cables. Why is this wire connected to earth?

Answer 1

Most of the equipment (electrical and electronic) has their large metallic body connected to this green wire from the plug.  In case there is a leak in the equipment or in the wire, then excess current flows in the equipment metallic cover.  That electric charge goes out into the Earth to a specific spot under the ground.

This point has a lot of capacity to absorb and hold leaked current.