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.