Why do we use a Washer?

The bolt on the Left presses down only on the area that the head covers.
The same bolt on the Right presses down on a larger area due to the washer between the Bolt Head and the material being bolted, the same bolt does a bigger job with a washer. Because it is a larger area the washer is distributing the pressure over a larger area.

Sometimes a washer can be used as a ‘spacer’ to fill a gap. For instance if the bolt has a thread that only goes so far, but not far enough, and it is two millimeters short a 3-4 millimeter thick washer can be used to make up the gap and the work is tightened.

If there is damage to the surface of the work where a bolt is a nice clean shiny washer can be used to cover over the damage and looks professional.

I have known of people drilling holes, putting in a bolt and washer where it is not needed just so that it covers a hole and still looks professional.

Some washer are made of plastic or rubber material. The rubber washer can seal a bolt and surface so no water gets through.

Some washers are used to help absorb vibrations, for instance the washer is made or part made from Sorbothane.

If in doubt, use a washer.