I deal with washers, in the development of precision mechanisms. They can serve a lot of useful purposes. For instance nuts and bolts can gouge into the parts t

I deal with washers, in the development of precision mechanisms. They can serve a lot of useful purposes. For instance nuts and bolts can gouge into the parts that they're holding together. A washer provides a sort of sliding bearing between the two, that takes the brunt of wear, and is easily replaced. It can spread out a load on a hole, including if the holes are made oversized to make up for tolerance variations in the parts. This is useful for bolting together wooden structures, or softer materials like aluminum. Stocking a variety of washers could be cheaper than stocking multiple types of bolt heads and nuts. It can be a spacer. There are specialized washers that can take up small angular tolerance variations, such as the spherical washers in some bike brake pads. You don't see them everywhere, and using them by habit isn't necessarily the right thing to do in all cases.