I believe that, (possibly only) in the early days, it offered to add any imported music to your library, which I think it indeed did by converting to AAC and putting in a directory structure of its choosing.
I went from meticulously curated MP3 collection on Linux to letting iTunes do it for me, and I realized that the meticulous curation and hunting down album art and everything was a hobby I didn't actually love. But if you do love it, great! Different tools for different jobs.
edit: I just compared iTunes 4 and Music in Tahoe and they still have the same preference! It's funny how they clearly did a s/iTunes/Music/, resulting in the "Music/Music" dir https://kalleboo.com/linked/itunes4-vs-musictahoe.png
Having iTunes organize your music into its own folder structure was an preference you could turn off, but that option was not shown in the onboarding so it ruined many a people's meticulously organized files...