I’ve been editing OSM for years. (896,339 edits in 3,427 changesets, apparently!) For me, it’s all about the free data. I once got a thank you note from someone who worked for a city with a particularly large municipal park. I’d added almost all the trails to the park and other information, and they’d used it to produce a printed map for the general public. Exactly the kind of thing I’d hoped for!
Personally, I do a lot of dualsport motorcycling and most backcountry maps around here are subpar. I map tons of trails and 2track and put them on the Garmin so I know where I’m going.
OSM is also great in lots of Europe–tons of detail.
JOSM is great.
Someone just recommended Organic Maps for the phone–it’s way snappier than Google Maps, but still not great with finding addresses.