Correct Bladepitch
The current will 'fill' every item that comes into contact with the wire, looking for an easier path. This is not the same as earthing/grounding, where it wants to get to the ground.
If the 'earth strap', for want of a better description, was not on, then the worker would get burnt as the current jumps/arcs.
The same gets done for helicopters when refuelling. What you want is easy current flow between the two and some times the helicopter and drum may be on trolleys and therefore not in contact with the earth. Remember if rubber can insulate, then so can car/truck tyres.