I've got stuff in Dropbox, OneDrive and Google Drive that only exists on their cloud and not on any devices - they're far more than simple sync services. True, if you are syncing a folder to your drive and delete from the computer it will delete from the service too (after all, that's what it's designed to do), but you can create / upload a file to a folder on the service that isn't synced anywhere and it will be fine exiting just in the cloud - that's what selective sync is for
For example, with Google Drive and OneDrive you can create documents using the web interface, edit them, etc and they don't need to go anywhere near a computer. You can sync your phone camera roll to Dropbox (and others) and never sync that folder to a computer; You can upload a file to a non-synced folder and then delete the file from your computer and the uploaded file will be just fine.