Reason for using washers in bolted connections are
The purpose of installing washers in a typical bolting system is to distribute the loads under bolt heads and nuts by providing a larger area under stress.
Otherwise, the bearing stress of bolts may exceed the bearing strength of the connecting materials and this leads to loss of preload of bolts and creeping of materials.
Alternatively, flanged fasteners instead of using washers could be adopted to achieve the same purpose.
