This part attaches a Raspberry Pi in a slightly stupid case I have to an external hard disk. There wasn't really a way to print it without at least some kind of support material...except leaning back at a 45 degree angle. This printed entirely without supports.
I built this printer around 11 years ago, it's a Folger 2020 i3, the company folded years ago. This is back before flexible PEI coated steel build plates were common; the bed assembly is a simple aluminum plate with a PCB heater suspended above it at the four corners by spring-loaded screws, and then the glass is binder clipped onto that. Glass was pretty much the meta for 3D printer build plates at the time because it's a perfectly flat material that's cheap and easy to source. The choice of a mirror over clear glass was mostly an aesthetic choice, though sometimes it can make it easier to manually level the bed, it makes it easier to see the gap between the nozzle and the bed.
This machine is pretty legacy by now and it's starting to show some signs of wear but it does still work.