Well....

If everything is good and healthy - i.e. no strange mishaps in its past - the washer should go between the dropout and the nut, protecting the dropout from the wear that the turning nut would otherwise cause when it makes contact with the dropout.

But if someone has been playing around with the cones and the over-locknut-distance, or maybe tweaked the fork a bit, anything is possible. The "proper" way to deal with sucha situation would have been to stick any washer used for that purpose inside the locknut, but kludges are common.