Washers

A washer’s primary purposes are to distribute a load, to act as a spacer, and serve as a locking device.
To compensate for an ill-suited fit: If a bolt is longer than the desired length, as many as three washers may be used to distribute the length of the bolt to a proper length.
To prevent galvanic corrosion: Galvanic corrosion is when one metal begins to corrode when it is in electrical contact with another metal. Washers act as a neutralizer and a separator between the two metals, and will prevent galvanic corrosion in the metals.
To bear a load: Washers help to distribute the weight of a bolt on another surface, and they will also act as a grip to keep the bolt in place securely.
To provide a shim: Shims are used whenever an adjustment is needed in order to keep measurements accurate. Washers are frequently used as shims in order to compensate for inaccuracies or other inconsistencies within a project.