The main purpose of a flat washer is to spread the force that is created by the tightening of a nut across a wider surface area.
Flat washers are also occasionally used as spacers, they are useful for this purpose if the bolt being used is slightly too long for the application in question.
An imperial washer can be used in conjunction with all types of non-metric threads including UNC, UNF, BSW, BSF, BA etc.1.44