A 58mm Metal Filter Stack Cap protects filters from dust and scratches while they are not being used. Simply screw your filters together, then screw these stack caps on the top and bottom. They're great for keeping dust, grime and fingerprints off the glass surfaces. Another benefit is that they keep all your filters organized in one place so you'll have access to them whenever you need them.
My 58mm Polarizing filter for my Micro 4/3 telephoto came in a huge plastic box that was designed for large filters and it took up so much space in my small camera bag that I frequently left it at home. With the stack caps, the filter takes up very little space so I take it with me all the time as I never know when I will be in a shooting situation that can benefit from the filter.
I've used filter stack caps with all of my cameras since the late 70s and wouldn't want to be without them.