[Cabnchassis]

I've worked in auto glass and have a close friend who owns an auto glass business. The original explanation given to Speedy is correct. The extractor often scratches the paint and lazy installers don't prime the scratches. If the window was done well the last time or hasnt been done a bunch of times it is a lot easier and usually leaves no scratches. End of the day it's a due diligence thing so if you don't trust your installer keep an eye on him or ask him to prime it anyway even if he doesn't think he scratched it. The trim covers the primer completely in most vehicles anyway.