Washers are used in between threaded fasteners, like a nut or bolt.
The washer acts as a means of distributing the stress applied to components that are bolted together.
The washer also cuts down on friction as the bolt is being turned.
It’s best to place a washer between two fasteners where the actual turning action is taking place when tightening.
A Spring washer is used when tension is required between the two fastening body.