Earlier quoted context omitted.
It's built on open data sets, including OpenStreetMap and OpenAddresses. In populated areas coverage is really good. Thanks to OpenAddresses, several countries have full address coverage. You can see the coverage map on openaddresses.io. New data gets added as these open data sources grow.
Why does OpenAddress need to exist separately from OSM? Can address geocoding data not be properly represented in OSM?
Many addresses exist in OSM. But there's a preference from the OSM community that data be entered by users familiar with their areas, that they can verify on the ground. Imports have happened in OSM, such as the controversial TIGER import in the US years ago, and the NYC building dataset more recently. But these require a lot of effort and attention to manage well.