MapOnMap is a tool where you can place your high detailed hiking map on top of an online map, i.e. an Overlay Map.
I have found it useful to have a tool, where I'm able to photograph a map, which I can navigate with the phone's GPS. It could be a map on an information board, a tourist guide map or a hiking map etc.
MapOnMap also supports Track Navigation. With MapOnMap you can record and navigate to a GPX-track. GPX-tracks is a standard format for describing tracks and can often be found on hiking sites.
These two main features makes it a perfect hiking navigation tool.