I really did not have any idea at all why. It just made sense when my graduate school professor in advanced design of metal structures explained it. The explanation was so simple I’m sure that the retention of the said info will last for my lifetime.
WASHERS ARE NECESSARY FOR SLIP CRITICAL CONNECTIONS TO HOLD THE JOINED MEMBERS WHERE IT IS SUSCEPTIBLE TO FAIL IN CYCLIC LOADING. THEY FIRMLY HOLD THE MEMBERS TOGETHER SO THAT THEY FUNCTION AS ONE.
For example, a certain member in a steel bridge truss is joined by bolts. Since traffic is a moving load, it generates cyclic loading which is the constant exchange of compressive and tensile stresses in the member. If the connection of joined members which is a weak link, is not designed and detailed properly, it will fail over time under these cyclic loadings. For a more simplistic example, let’s say you’re holding a tie wire (alambre) and you repeatedly bend it back and forth. First it will generate heat, it loses strength in the process until it ruptures.
As the bolt is tightened by applying torque to the head and the nut (the head and the nut are always hexagonal in shape, that is, it has 6 sides) it generates a tensile force in the bolt while the washers provide the surface required for a clamping action (it presses the joined members firmly together) and prevents sliding by means of friction in the base metal.
Simple as it looks, bolt washers play a very vital role in the health of the overall structure where they are employed.