Library music, also known as ‘production music’, is created mainly for video professionals working on TV shows, movie trailers, advertising, and more. It’s not written for specific visuals but, instead, too (hopefully inspiring) album concepts, and distributed around the world where it can end up being be used in random ways.
Credit to source of information;
https://www.soundonsound.com/music-business/all-about-library-music-part-1
In short, library music is the ‘last resort’ for production companies that are in need of music, they can head to a library and search through thousands of tracks that are put in to be used by companies to they can earn royalties for their work, if the artist gets lucky, a big company will use the created audio and they may earn royalties every time the music is played through media. Royalties were established so lessor known creators didn’t get their work borderline stolen, and so credit could still go to their work and they can earn alongside the media.
I watched this video that explains that concept.