Hi Jan --no one has answered this for you,
The reason for a washer is , mainly two reasons ,first when final
adjustment of the bolt is to allow a slight movement against the washer, rather than
a solid feel against solid steel.
The Second main reason is with a washer ,this allows a very small clearance between
the two steel arms being tightened,so gives clearance when the two arms are moved
past each other when lifting top up and down,and over time allows dirt and grime to clear
the arms.
Another example and not many owners will realize this, is our very rare Heyco 300 Pliers
that we have in our tool kit, also have a very fine thin cupped washer to help slide the adjustment
open and closed,.it is under the half round side head of the back of the pliers.