i will explain in detail. Question you need to ask yourself is when and why is navigation guidance mode icon pressed? Two fold answer to it.

one when we are done Routing our route, enabled Tracking settings, GPS is locked-in, a certain Zoom-level is set on map, Download Map Tiles in cache done (using offline routing mode), clicked on Offline Mode and now you are all set on your journey from a Source -> Destination; point being all the settings take place prior to navigating and they are very nicely laid out in drop down menu.

two, this mode is used when we are driving or perhaps biking etc i.e. navigating our way. We are on the route now. We have set the controls and now we want marble to take auto-pilot and guide us while we concentrate on the road. By toggling to full-screen it gives a clear, unclustered and bigger map view in range making it easier to peek and know which next turn is coming up. Also forcing a touch to ideally either pull a car on the side or stop biking before making any necessary changes/adjustments to the route.

Lastly, in my last post i mentioned a fullscreen icon which i also told earthwings on irc chat. That would give freedom to use however one feels fit.