It's simply good practice. I like washers under both the nut and the bolt head. I've seen cases where they weren't used and some @#$@%^& person actually drove the nut or bolt head partially into the flange. A real joy when you have to take it apart. Also recommended when using lock washers in the order flat washer, lock washer then nut. It gives a better bearing surface for the lock washer and nut.
