The Full View button allows a user to quickly return to the default view (extent) of the map image. When the map image is rendered for the first time it is at its default view. A user can then perform several operations that may alter that extent (zoom in, zoom out, pan, etc...)
Returning to the default (Full View) extent will honor the current map configuration. For example, a user may zoom in on a map, turn on a few desired layers and Refresh the map, then return to the previous default view by clicking the Full View button in the toolbar. Doing so will re-draw the map at the default extent (Full View) and maintain the layer visibility just set by the user.
It is important to note that layers are still restricted by Scale Dependency (if applicable) when moving to Full View.