Q: Photo's Library backup

Why would I still need a Photo's Library backup, whilst all my photo's are automatically saved in iCloud so accessible any time anywhere?

For three reasons:

1. The copy in iCloud is the central storage, synced with all devices.  Any changes to the iCloud photo library will be made to all copies on all devices. If you accidentally delete a photo, or a photo gets corrupted by transmission errors, it will be damaged in all copies on all devices. You cannot restore a damaged photo from iCloud. You need a separate copy of the library that is not synced with iCloud.
2. iCloud Photo Library is no archival backup. You may want to restore photos, that you deleted some time ago, because you are running out of cloud storage, but want back later.
3. The iCloud Photo Library is only accessible by network and tied to your AppleID. There may be times, when you find yourself without network access, but need your photos. Or a problem with your AppleID may prevent you from signing into iCloud. Then you will need a local copy.

You should alway have a copy of your photos, that you can always access and control, independent of network issues. Store a copy of the library on an external drive and store it in a strongbox, if you value your photos.

See:   Back up your Photos library

Couple of reasons:

1. The internet may be always there. But will you always have access? ISPs can go down...

2. What if Apple have a massive server crash?