I put masks in the same league as car seat belts or safety glasses - most of the time they don't make a lot of difference, but when they do, it's a significant difference.

In this respect they're more like safety glasses or other PPE: if seat belts keep you from going through the windscreen you'll know all about it, but if safety glasses save you a trip to A&E with something in your eye, it won't necessarily be obvious.