Dropbox may have its uses. It can be used as a backup, for instance.
Except for the mobile backups, iCloud is not meant as a backup service. The reason we use iCloud Photos and iCloud Drive is so that this makes our stuff available on all of our devices.
iCloud Photos is a synchronization service. When you engage iCloud Photos on a device, then the Library on that device is kept exactly the same as the iCloud Photos Library. So, for instance, if you take a picture with your iPhone, it is added to the iPhone's Photos Library, copied to iCloud Photos Library, and then copied to the Photos Library on each of the other devices that you have connected. If you delete a picture on your Mac, then that picture is deleted at iCloud and on all the other devices.
So you can't think of iCloud Photos as a backup service, since it backs up mistakes, and there's no way to undo them. Whatever you do with Photos on your device-- it happens everywhere else.
iCloud also offers iCloud Backup for mobile devices. But iCloud Backup doesn't include pictures if they are already being copied at iCloud Photos.
So we keep backups on external drives or, perhaps, in other internet services, like Dropbox. I don't use Dropbox.