Yes, that’s an interesting question since most mobile devices come with gigabytes of so-called free cloud storage for safeguarding your files. The main purpose of such services, however, is to allow you to access your collection of documents, music, photos, etc., from any of your devices and see the same stuff. For example, on an iPhone when you add a photo, it will appear in the Photos app on your Mac. But deleting a photo in either device’s Photos app will remove it from both (because removed from cloud storage — which is reflected on each device).
Syncing may be all that you need in many cases, especially if you are disciplined in tending your collection or you purchase so much storage that you never delete anything. But otherwise you probably want the assurance of a restoration timeline like in Apple’s Time Machine app — you want versions of your collection, whether stuff was deleted later or not.
Reference: PC World, January 31, 2018, “The best free backup software and services: Reviews and buying advice for protecting your data.”
… you say: I have free online storage through Apple’s iCloud or Google Drive, or Microsoft OneDrive. Then there are services like Dropbox, with 5GB for free.
The issue with all those services is that they’re not necessarily true backup, but syncing. That is, when you delete a file from any device or online, it’s deleted from every device. Lord help you if you make a mistake and don’t realize it in time. True backup means retaining data indefinitely no matter what’s happening with the data elsewhere.
Personally, while I have files stored in iCloud, Google Drive, OneDrive, and Dropbox, for local backup of my Mac I use SuperDuper and Time Machine and for my PCs File History and System Image backup.