Theoretically the flow could cause a static charge to build up on the plug. The voltage can become high enough so that it causes an arc to a grounded component. The arc may be inside the body where it could eventually cause erosion,ignition of the product, or it could be outside the body where it could trigger an explosion or become a direct hazard to personnel.
An antistatic device ensures a positive electrical connection between the body/pipeline and the plug so that the static charge can never accumulate.
Antistatic devices are much more necessary on rotary valves where the closure element (Ball, etc) is frequently floating on nonconductive polymer seats. Usually nothing more elaborate than a spring washer on the valve stem is required to drain the accumulated electrons.
