These are little bench dog flats. They fill in the bench dog holes on a typical workbench (in my case, a Harbor Freight 60 inch, 4 drawer Hardwood Workbench. Bench dogs are used to hold clamps or provide support to clamps, but sometimes small parts can fall through the holes. The bench dog flat simply fills the hole.
I printed one to make sure it would fit, then I printed a set of 13 more to provide one bench dog flat for each hole in the workbench. 10 percent infill is all you need.