Spoke washers stabilize the spoke head in situations where the spoke does not fit perfectly in the flange due to differences in diameter between the drilling and the spoke or because modern spokes are not designed to work with narrower flange widths as seen with older SA hubs.

Old steel shells did not need have flanges thick as alloy hubs to get the same degree of strength and vintage spokes have shorter bends... modern spokes are designed for the wider flanges on alloy hubs and for a time DT Swiss made their spokes with a longer elbow to facilitate faster wheel builds.

This was a bad idea as then the spoke would not lay along the flange properly without the use of a spoke washer.

They cost about 2 cents each so you are looking at less than 75 cents in extra cost if you are building an entire wheel.