I haven't seen this feature in players yet but think that all will want it.
the feature is to let the users see that they are in the right aspect ratio.
like the divx icon that apears at the beginning of a clip (if the user enabled it) there can be a square that appears at every movie that lets the user see if the square is equal on all edges the aspect ratio is ok.
if the square is not equal than the aspect is wrong.