This was the reason I chose Spotify over Apple music all those years ago. Any reason to prefer Apple Music over Spotify? Spotify's recommendations have been absolutely stellar for me over the years and I have a lot of playlists and stuff "locking" me in.
With iCloud, tracks you add to your library are automatically uploaded to Apple's cloud and become available on all devices. That helps a lot because you don't have to resort to using some other player for that those obscure albums that aren't streaming anywhere. After all, while Apple and Spotify have a lot of music, there are still many holes in their inventories.
Apple has always been much nicer about offline track availability. Just click the download icon and the tracks will stay on your device. Spotify has had this feature, but it's been flaky. After Apple Music launched, they eventually added a "Download" toggle to albums, but only in the mobile app (it's there for playlists in the desktop app, for some reason).
Spotify has a 10,000 song limit that applies to adding (or "liking" as it's now called) to your library. You can keep more in playlists, but you can't "like" more than 10,000 songs, which is crazy. It's not a lot of songs. My jazz collection alone is more than that. Apple's limit is 100,000, as far as I can tell.