The Edges (the cycling-specific models) can produce turn instructions for tracks. I believe that's a feature that only the cycling (and, possibly, the motorcycle-focused Zumo models) have.

Other models can generate turn instructions for "route" files, which have a few waypoints.

The cycling models can't use route files. This doesn't make much sense to me (maybe, it's to make things less complicated for people).

I believe websites like Ride with GPS were created to serve the people using the cycling-specific Garmins.