The late Jobst Brandt felt that a washer between the pedal axle and crank arm could reduce fretting damage to the crank arm, which can lead to failure at the pedal eye. He did not think that a washer was an ideal solution to this issue, though, and modified cranks to have a conical seat for the pedal axle, similar to those used on automobile lug nuts. That modification would also eliminate the need for left-hand pedal threads on the left crank arm, but it's not a modification that can be done in the typical home workshop.

That said, pedal washers are inexpensive and IMO any reduction in fretting damage has to be a Good Thing. I fail to see a downside to using pedal washers, so I put them on all my pedals.