Can I get an explanation from one of the HVAC experts on this one?
My answers are based mostly on the ICC codes. Advice given is my personal opinion and every person performing work should acquire a permit from his/her jurisdiction and get the work inspected. My opinions are not directions to follow for DIYs or professionals
I think someone had a flying bug/insect problem.
Or they were trying to reduce wind issues.
1. It's a flammable item on a heated exhaust.
2. It could interfere with the opening and closing of the damper.
3. There is a small hole in the side near where it is screwed on. This would allow bugs to enter.
Important note Ė I donít know man made laws, just laws of physics
Disclaimer: I'm a big fan of Darwin awards.