It's one thing I seem to use a lot onboard. Looking for small leaks on the backside of hoses and fittings, checking the fit of something, looking behind a settee for a lost item, there seems to be a zillion places on a boat where you can't see the backside of something.

Maybe you already have one. If not, you might want one.

I don't think you have a wife to give you one so you may have to find something else. About the size of a playing card seems good.