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I'm guessing that a soft aluminium or plastic washer will stop a steel pedal spindle damaging a carbon fibre crank, or will prevent dissimilar metal electrolytic corrosion affecting an aluminium crank.
Is that it ?
I think it stops the peddle eating the crank, should also help getting the torque right as a better surface to tighten onto
I use steel washers to stop the crank being damaged. It's not so much of an issue if the spindle has a round end to it, but it's not uncommon for the spindle to have two flat sides for spanner fitting, in which case they can dig into the crank.
I also use washers on caliper bolts to stop them digging into the caliper, which can make realignment something of a headache if the surface is damaged by a bolt head.
Can help to stop cracks from starting as the hardened [insert axle material of choice] bites into the face of the crank arm. For example TA cranks always come with pedal washers.