In the travel packages the checkbox "Use Preferred Path" seems to do nothing to affect the package. What's it for?

If you hit Ctrl+W when looking at any cell or worldspace in the Creation Kit's render window, you'll see color coded NavMesh triangles. Orange is preferred to red, as the AI sees it, so that flag would increase the likelihood of the NPC taking the long route rather than roughing it through the bush. Think of real sidewalks as orange or preferred and roads as red; you'd sooner walk on the sidewalk though you could walk in the middle of the road.