Why do vehicles carrying inflammable material and running on rubber tyres, always drag a chain along the ground?

When the vehicle is moving, due to friction in air, the body of the vehicle gets charged. If this accumulated charge becomes excessive sparking may occur and inflammable material will catch fire. Hence, mettalic chains dragging the ground is introduced so that the excessive charge gets leaked onto the ground and fire can be avoided.
Similarly, vehicles running on rubber(insulator) tyres also drag a chain along the ground so as to dissipate the static electricity that has been accumulated on tyres to the ground.