if the music is older, and not from the US, it’s often not on spotify. Versions matter too - even for some mainstream bands their B sides/acoustic/live versions just aren’t on spotify or youtube. Album metadata for spotify is garbage too - it just isn’t an adequate replacement for a record collection.
I do use a spotify subscription, but for me it’s a tool for playlist generation and music discovery.
Also audio quality, as others have mentioned.
I think this is the use case for Kodi, if you have local media. If you also want multiple device sync without hassle, then Plex, but this is an online account. I hear good things about Jellyfin, which is a FOSS alternative to Plex.