The problem seems to be that people are expecting something very different from iCloud, a separate external Cloud storage or a cloud backup, as soon as they are hearing "Cloud".
What Apple is offering is much more and perfect for people who are using several Apple devices with different system versions. I am using a collection of five different Macs with different versions of macOS, two iPads and an iPhone and an iPod Touch. I can work with my photos on any of my devices and the Photos Library in iCloud is mirrored on all devices, perfectly in sync. I can use any device that I am currently having with me to work with the photos and continue on a different device, when I get home. You can use iCloud to save storage on your devices, as David McKinley pointed out. Just use the optimize Storage option, and Photos will automatically remove photos that you have not used in a long time and keep just a smaller thumbnail for browsing. But do not delete any photos you want to keep, let the system do it for you. With optimize Storage enabled, you can stop worrying about the storage used - even if your storage will always appear to be full, the system will free storage for your, when you need it.
I am enjoying iCloud Photos primarily for the syncing capability and avoiding "Optimize storage" like the plague. I prefer to have my photos stored locally, so I can access them easily, if the internet is slow or not reliably, or when I am completely out of reach of a fast, reliable or affordable internet connection. Keeping the originals mirrored locally is also making it easier to make regular backups of the Photos Library. An optimized Library would be incomplete, with some of the original media files missing, so Time Machine can only backup the structure, but not the photos and videos. I prefer to pay for more onboard storage on my devices instead of for download fees for downloading the same photos and videos over and over again.
Three Good Reasons for Using iCloud Photos Library and when not to use it - Apple Community
