One example use case would be me and my music collection - A fair portion of my digitized music was ripped from my CD collection perhaps 10 years ago, and back then ID3 tagging was fairly rudimentary, and not even provided by many CD rippers.
So, I have the majority of my music collection sitting in per-artist folders of my own creation, with incomplete/no ID3 tags. Using playlist-based players always results in my music being sorted horribly badly as a result, trying to play music by a particular artist will always end up with it being split between the artist's name and "unknown", possibly with variations on the artist's name in some cases, with different capitalisations, dashes instead of spaces, etc etc.
In other words, the use-case is for people like me who have their music collections sorted the way they want it, without having spent countless hours making sure every single track has correct ID3 tagging in place, and just wants to listen to some music by X or Y artist.