I have an offline music library that I use with Apple music app. It's the only way I can have Siri play playlists by name when using CarPlay; despite this functionality being broken for me since iOS 17 (have a longstanding bug report with escalation with engineers that have been saying they'd fix the bug for about 2 years now). Synchronisation is awful because it can't reconcile multiple copies of the library on different devices, so you're essentially tethered to one mac (or windows) for synchronisation.