I got that crap on my 2010 GMC truck frame. That was one thing they "touted" that they had a factory wax coating on the frame. Which IDGAF since I live in South Carolina. Big rust risk here. haha But I get it. If you're up north, you can appreciate anything that's stuck to the frame that won't trap moisture and repels it. When I was stationed on the sub going through overhaul in the Portsmouth Navel Shipyard in Kittery, Maine, I saw all sorts of horrid rust on fairly new cars at the time. Scary! If you had a southern car with no rust up that way, you could sell it for a pretty penny. Maybe that's why they didn't care if you had a title for a car over 10 years old in NH at that time...
More G-body frames have rusted out behind the rear wheels than anyone would like. Shame actually. The 87 got some sort of rustproofing when new but I haven't really delved into that too much, but it was a good way to document the mileage along the way when they did their annual "checkup" when the original owner had it. One year I think the car was driven less than 500 miles or some crazy crap. My 85 442 got the brown looking dealer goo/wax like crap. Again, living in SC it was kind of stupid IMO. That's coming off when I get the body off the frame.