Pennsylvania has two inspections, they are done at the same time. One is an emmissions check. The other is a safety inspection to make sure your vehicle is road worthy. The check for tread depth, sufficient brake pad surface remaining, worn suspension components, wipers, all lights functional, complience with motor vehicle law.
It's supposed to keep unsafe vehicles off the roads. A responsible driver has no problems as they would get problems corrected themselves. It's the morons with no brake lights, bald tires, rotted exhausts they want to weed out.
In actual fact it's a source of revenue for garages. The problem is with some that try and gouge a customer.