If you do anything with old Macs then sometimes you have to take the back off. The stamped steel that the chassis is made from can be really sharp and will scratch up your desk! Even if it isn't a fine surface you care about you still don't watch to hear it scratching as you turn the machine around. These foot units clip over the scratchy metal edges and will stay put while you work on the Mac. Depending on whether you still have the RF shield in place you might even be able to leave the feet on the Mac permanently.
edit: I can no longer edit the original file due to Solidworks licensing - great package but my current university chooses Inventor, sadly - but I have split them in Prusa Slicer, always a good option when you just need to separate parts. Combining parts in the same STL was a very bad habit and I don't do it any more!
The author marked this model as their own original creation.