Washers are designed to spread out the force of the bolt head over a larger area so that the bolt can be done up to a tighter torque without damaging the contac

Washers are designed to spread out the force of the bolt head over a larger area so that the bolt can be done up to a tighter torque without damaging the contact area. They are actually quite malleable, which is kind of required because the contact area could be imperfect or ever so slightly angled. A bolt on the other hand, there is almost no malleability there at all. Most bolts are rigid and inflexible because of their geometry and because the tiny threads need to be very strong. In recent years, washers have evolved a little further and now many washers also include a locking feature to prevent the bolt from coming undone. You also see thicker washers and spring washers which are designed to regulate the bolt force even if the temperature varies a lot and the bolt expands or contracts. Finally, the washer material might be important. Steel bolts corrode against everything, so most washers that you find are probably zinc plated or similar and designed to insulate the bolt head material from the contact surface material.