The idea is to allow the level symbol visible in a view to be overridden. The Level Type would still have a Symbol associated with it, but it would be the default symbol.
REASON: Much of the time in views, it makes sense for the symbol to show the elevation with the level name, but often using a different symbol can be preferable in some views on drawings - say one with just the elevation, or with a different arrow shape (or whatever you want to call it). As it stands, when the symbol is changed on the Type, it changes throughout the model.
You would be able to reset it later to the <default> symbol as set by the Level Type.