iCloud is not primarily a backup. It is meant to keep the documents and data on all your devices updated. If you are using iCloud Photo Library and edit a photo on your Mac, you will see the same edits on your other macs or your iPhone or your iPad. Import a photo on one device, and it will appear on your other devices. Delete a photo on one device, and it will be gone from all device. Your iCloud Photo Library will be identical on all devices. If you enable the option "Optimize Storage" on one of your devices, this device will not store full versions of your photos, only smaller versions. You can browse the photos, but will need an internet connection to edit the photos. But without "optimize storage", you will have a full working copy of your Photos Library on your device and can work with the photos with out Internet.
I believe I lose the ability to hear music that I used to have on my machine if it is in the cloud but that might be because of copy right issues.
With iTunes Match you can keep all your music stored locally as well. With apple Music you can download the music you want to store for offline listening.
Your Mac will keep the Amil messages, that you already y downloaded and you can read them offline.
But remember, iCloud is not a backup. It is your central working storage. Any changes are immediate. You still need to keep local backups as well.
