So let's say you have a folder called Geneology on your computer. It contains 45 photos that you've scanned and edited. They have never been on your iPhone or iPad. How do you get this as a folder onto your iPad?
If it's an operating system folder, as opposed to a Photos app folder, then it won't work. The iCloud Photo Library syncing is for photos and videos in the Photos app, and it will preserve the folder and album structure which will be synced across all your devices. Copying operating system folders to iCloud Drive will not sync them with your devices.