- import (adding object to database) - for example https://budynki.openstreetmap.org.pl/ is used by OSM community in Poland to import official address and building data after review (sadly, quality makes necessary to review data but still saves time overall)
- using for mapping (for example Bing aerial, Strava heatmap, aerial imagery released by many governments is also available on a compatible license)
- linking (for example many OSM objects link Wikipedia articles about them or their official website)
- used to find places for mapping (if Google starts showing new road I am not allowed to copy it into OSM - both due to licensing and low quality of Google data. But I can select this place for the next bicycle trip and map everything there as I survey this area)
Not sure how it matches general terminology, OSM has some slang/terms which is sometime incompatible or unknown elsewhere.
Vast majority of time is used up on handling useful but not fully correct data.
I am not aware of any datasets that are high quality, on a compatible license, including data of interest to OSM and existing for longer than a year - that would not be imported already.
There are more people interested in making imports than high quality datasets on a compatible licenses, this is not the bottleneck.