Mr. Shimano knew what he was doing, so leave it be! That washer serves as a 'lubricant' between those serrated edges, so they don't wear each other out.

Without the washer, the serrated edges will eventually wear each other smooth, and your lockring won't.

[Edit: the serrated edges of the lockring and outer sprocket will only wear each other out if you remove and reinstall the lockring several times for cleaning or gear changes. If you have already removed the washer, but intend to leave that cassette on until it is completely worn out, there is no need to reinstall the washer.]