In my area at least, I've found that even where the road networks are complete there are usually a lot of things that can be done to aid navigation. Some examples:

Walking: Tag roads for sidewalks, add footpaths and crosswalks.

Cycling: Tag roads for bike lanes, add bike paths.

Driving: Tag roads for turn lanes, and tag intersections for turn restrictions.

And of course POIs are always missing or need updating or additional info.