One of the few things I absolutely need iTunes for is that some of the music, apps, and other iTunes content I’ve purchased over the years is no longer available through iTunes and has also been removed from my purchase history. In this special case, syncing with iTunes on my computer is necessary to transfer this content from the iTunes library on my computer to my iOS devices.
iCloud Music Library (used to be iTunes Match) would probably get most or all of it. But it typically needs more storage than the free 5GB per-month, so it can cost.
How iCloud Music Library is suppoed to work.
It looks at your music library, and figures out which songs were not purchased in iTunes. If the song is available in the iTunes Store, it makes it available to your iOS devices. This is often a better quality copy than the one in the original library (unless you are using lossless files). If the song is not in the iTunes Store, then it uploads that song to iCloud, and makes it available to your iOS devices.
It does not erase the original tracks from your computer or iOS device unless you choose the Delete & Replace option when turing the service on (for that device). Think carefully about where you choose Keep or Delete&Replace. You can lose music, or end up with duplicate tracks if you choose poorly.
I’ve known it to mess up now and then, but mostly it got it right when I switched. To be on the safe side, a backup of the computer is a good idea. That way you can restore the old library if something goes wrong.
The main difference is that iCloud Music Library streams by default. You have to explicitly download songs to the iOS device if you want them stored locally, available without an internet connection. Files synced with iTunes are always local.
Turn on iCloud Music Library