PVC conduit is a polymer made from Poly (vinyl chloride) which can be a considered a plastic (depending on who you ask). So when you have large quantities of air moving past this material it builds up static electricity. If this is not earthed and allowed to build up, the static electricity can cause a spark in the ducting. This is especially important to industries that are working with combustible materials such as timber or paper processing mills. If you add a spark to a combustible material and high volumes of oxygen, then I am sure we can all imagine what happens next.
This very principle is also why DCS have blast gates installed on all of our collectors, to ensure that any build up pressure from an explosion does not turn a dust collector into a ticking time bomb. This is also a reason why cutting corners with choosing the right dust collector can have a massive detrimental effect for your staff and business.