Meredith723 wrote: … what is the point of iCloud for storage if it’s synced
"Synced" means "kept exactly the same." Using iCloud Photos synchronizes your Photos Library with your iCloud Photos Library. Having iCloud Photos turned on makes the pictures in your phone's Library exactly the same as pictures in your Library at iCloud.com. If you add a picture to one device, it will appear on the others. If you delete a picture on your iPhone, it will be deleted at iCloud.com and on any other devices you have. Of course, by deleted we mean that it can still be recovered within 30 days unless you delete it a second time in the Recently Deleted view.
Optimize Storage saves space on your phone by keeping only smaller pictures there, and relying on the full files being kept at iCloud Photos. Then, when you need the full pictures for printing or editing, etc, the phone grabs the full file from iCloud. This is very cool, but it can lead to a slight delay, sometimes. With Optimize Storage, you save maybe 80% of the storage that would be used otherwise.
If your iPhone storage is very full, Photos may have a problem scanning pictures and making adjustments to optimize your storage. It's possible that you may need to erase some apps or other files to make room for Photos to move around.