The difference is that in Dropbox you can have files that only are in the cloud, but have basically aliases in finder on your desktop that you can click to automatically download them. This way you can have a cloud storage LARGER than your local storage on some machines, but in full on others. With iCloud, unless I'm mistaken, all the files in the iCloud drive are mirrored onto your local drive. I like to keep some files off of my laptops (but on my different desktops), but still easily accessible when needed when needing to update them or give them to somebody. It is all about managing different space needs on different computers that are all sharing the cloud drive files. If it was only for backup, that would be no problem as backups are not accessed that often.Wasn’t this the big difference between iCloud Documents and Data (pre-2014) and iCloud Drive (post-2013)?
As far as I’m aware, iCloud Drive is its own section in the Finder, separate from the rest of the system. Does this not work the same way as Selective Sync? Is selective sync closer to how Windows handles OneDrive files in File Explorer? Couldn’t Optimize Storage act the same way on a system-level in macOS?