It will remove any amount of lye that might have been carried over my the DMT molecules (won't be much).  There might be more to it than that, but it's always worked for me.  It is pointless to do on the actual crystals, but works well when washing DMT laden Naphtha.

There is a difference between washed and unwashed  in how harsh they are...at least that has been my experience.

Not sure exactly why, but sodium bicarb washes seem to take some of the yellow out as well.