I contribute to OSM so that the maps made with it are more accurate and robust. What do I use the maps for?
All the better hiking apps I know of use OSM data so contributing to OSM means the maps I use when hiking are better.
There are a number of general navigation apps that used downloaded OSM data (OsmAnd, Organic Maps, Magic Earth, etc.) At least a couple of these (OsmAnd and Organic Maps, not sure about Magic Earth) are very privacy respecting and all of them can be used without having a data connection. So if you either are cheap and don’t use mobile data or you are in areas with little or now mobile coverage your navigation still works.