Washer screws are used for multiple purposes when mechanical parts are assembled. Here are some usages for washer screws. Some washers have a special design that attempts to help keep the nut and bolt from coming loose. It could be used under the bolt head in instances where the bolt screws into threads in one part of the assembly. A washer is sometimes used to protect the surface of the assembled parts. A nut or bolt head being turned during the tightening process can mar the part surface around the hole and a washer can be used to take the abuse as opposed to the part. This may be particularly applicable when the parts are a softer material such as plastic, brass or aluminum and a washer made of a harder material is used. Sometimes a thin material is part of the assembly and a larger diameter washer is used to help distribute the pressure of the tight bolt across a larger part of the thin material.