The grounding strap prevents static charging of the truck.
Static charge can damage the vehicle electronics or lead to unpleasant electric shocks when touching the vehicle. This is particularly common when using non-marking tyres, since the light-colored rubber compound contains silicate instead of carbon, which means it is less efficient at dissipating tension into the ground.
A grounding strap consists of particularly good conductive material. It can easily be mounted under the forklift and creates safety.