Washers are used to provide a leak proof sealing when there is a liquid.

Washers also facilitate for tolerance when there is a small deviation in the screw head size.(but hardly in low stress static applications).

Standard washers have two main purposes: To distribute the pressure of the nut evenly over the surface, so that the surface isn't damaged, and. To ensure that the nut is pressed against a smooth surface, reducing the chance that it'll gradually loosen because it's in contact with an uneven surface.

It is also use to seal the area to avoid leakage in higher pressure system which maybe liquid or air.