It’s not so much about filling it up, but when someone goes to eventually renovate the place lol. Open the drywall and just have a bunch of blades to clean up… Or if you get a leak and have to now deal with a puddle of rusty blades.

I want to say that possibly one of the medicine cabinets had a smaller container that collected them at some point, but again, it was still fixed behind the wall lol.