Different washers have different purposes. Some, as already posted, are mainly there to protect the underlying surface against the turning nut/bolt. Some are used mainly to distribute the load over a wider surface when working with a compressible material. Then there's a family of locking washers whose job it is to prevent the nut/bolt from unscrewing itself due to vibraation and whatnot. For IG hubs there are anti-rotation washers whose task is to prevent the axle from turning as the gears brace against it. For some pedals, hubs and certain threaded forks there are tabbed washers to isolate the movement of one nut/cone against a locknut. I thing I've seen an Ashtabula crank in that design too. When you get into the outer reaches of the washer territory you can come up against odd beasts as spring washers calibrated to keep a certain tension on the bolt/screw, thin washers doubling as shims, thick washers doubling as spacers and non-metallic washers for electrical insulation or to prevent corrosion between dissimilar metals.