Let me try to explain this in a different way; long-time users (Harry, et. atl.), feel free to correct me on any of this.
Erik, yes, you _can_ use the online map features for routing. However, first please understand that the online map services are separate organizations from OsmAnd, and might not have recently-updated map data. Some are known to not update their map data for months, or even years. The OsmAnd developers cannot make them provide more recent updates; they are simply different companies, each with their own goals. Why are they still offered in OsmAnd? Because many users still want the ability, even with outdated map data.
Within OsmAnd, the base World Overview Map is exactly that--an *overview* map of the world. It only provides the most important highways as set in the OpenStreetMap database. It does not include most local roads and streets, such as in residential areas where people live. Using 3rd party data along with tools provided by the OsmAnd developers, you can generate a complete world road map with complete local road details, but the effort would take a few weeks, and require dozens of gigabytes of data..
In order to get the most capability from OsmAnd, you really do need to download the offline area maps (nation/country, state, region) where you live, and want to travel through/to, and use OsmAnd's internal routing function. You can still use the offline maps with the online routers, and get a reasonably accurate route to your destination, but the offline maps are still needed. For myself, I have found that the offline maps and OsmAnd's internal routing engine usually generate good navigation data, often comparable to specialized routing applications such as Waze (without the live traffic information that Waze and Google have access to). Of course, OsmAnd's route abilities are totally dependent on accurate information in the OSM map database; I've encountered several situations where OsmAnd *shouldn't* have routed me a particular way, but the fault was in OSM, not OsmAnd. And also, in several of those cases, a comparable route from Google also sent me down bad roads, because their map database also had poor data. Then there's this recent routing fiasco in the United States from Google:
https://denver.cbslocal.com/2019/06/27/google-maps-detour-drivers-muddy-field-denver-airport/
The OsmAnd developers are often tweaking their routing engine, and so sometimes the route generated by one version of the app may differ slightly from the one generated by another version, but they are usually similar.
--jack