Three Good Reasons for Using iCloud Photos and When Not to Use it

Occasionally users expect iCloud Photos (Library) to be a full backup of their photos and also a way to move the complete Photos Library off their devices into the iCloud.

iCloud Photos is not a backup of your photos, and it is also not an independent external storage but iCloud Photos is primarily a syncing service, and perfect, if you are using Apple software on many devices with different system versions.

Do not use iCloud Photos,

• if you want to back up your Photos Library. An optimized iCloud Photos library is even harder to back up.
• If you want to remove the Photos Library from your device completely. An iCloud Photos Library will still need some storage on your device.

You should use iCloud Photos for three reasons:

1. Syncing your devices. That is the primary purpose of iCloud Photos, to keep the Photos Library identical on all your devices. iCloud is keeping a copy of the current state of your Photos Library in iCloud as the master copy. When you modify your photos library on any device, it will be updated in iCloud and sync to all your devices. If you delete photos on your iPhone, the deletion will be updated in iCloud and on all other synced devices.
2. Saving Storage: To save storage on your device, enable optimise "iPhone Storage" or "optimise mac Storage". iCloud Photos will automatically remove full resolution versions from your device and replace them by light-weight, optimised versions. But while using iCloud Photos you must never, ever delete a photo, that you want to keep. Leave the space saving to the automatic "optimise" feature. The photos on your device will still use 10% to 20% of the storage used in iCloud. You cannot get the photos completely off your device with iCloud Photos Library. The device with the smallest amount of available storage will determine, how large the iCloud Photos Library can grow in iCloud. For example, if one of your devices is an iPhone with only 64 GB of storage, you should not let the iCloud Photos Library become larger than perhaps 500GB, or even the optimized photos will not fit onto the iPhone.
3. Off-Site Storage: iCloud Photos is not an archival backup, it does not offer a history, but what you will get, is an identical copy of the current state of your Photos Library in an off-site storage. If your device is lost or stolen, or damaged, or the house burns down with all your backup drives, you can recover the current state of your Photos Library from iCloud Photos to a different device. This will protect you, if your device is lost, but will not protect you from user errors.

If you deleted photos from one of your devices while using iCloud Photos, you cannot recover the deleted photos from iCloud Photos, because the iCloud Photos library will always be kept identical to the current state of the Photos Library on the Mac, iPad, iPhone or other synced devices. The photos and videos you are seeing on any of your devices are the same ones that are having in iCloud.

The Recently Deleted album is the only safety line provided by Photos to recover deleted photos. Once the items have been removed from Recently Deleted, they have been permanently deleted.

If you are using iCloud to store photos, videos, documents, and data, you need to make regular backups of your photos and all other documents and data stored in iCloud. Make backups of all iCloud items on a computer, or you will not be able to recover items you have accidentally deleted, once they are no longer in Recently Deleted. See: Archive or make copies of the information you store in iCloud – Apple Support

You are probably making regular iCloud backups of your iPhone or iPad to save the current state and to be able to restore the current state ofter a factory reset. The iCloud backup of the iPhone or iPad will usually include your photos and videos as well. ( What does iCloud back up? - Apple Support)

But this is no longer the case, if you are using iCloud Photos. Items already in iCloud are not included in the iCloud backup of your iOS devices.

For a Mac, the Time Machine backup will include the photos and videos, that are stored locally on your Mac. As long as you are not using "Optimise Mac Storage" for your iCloud Photos Library, the Time Machine backup will include the Photos Library. The Time Machine backup for an optimised Photos Library will be incomplete and you need to backup the original image files separately, see: How to back up an optimized iCloud Photos Library - Apple Community