As a factory routine test, PD is critical, as it will tell the purchaser how well the resin has been cast and cured (it is easy to skimp on resin content and have unacceptable curing temperatures, as well as not having sufficient vacuum pressures during casting)
Any anomolies in the resin (ie voids, holes etc) can be picked up using a PD test (if it is done correctly).
Past this point, as Zogzog points out, it can be an indicator of deterioration of the insulation, or an indicator of surface cleanliness.
Have a look at the SGB website. They have examples of online monitoring systems if you want to go to that extent.
Phil