You find out there was no backup after you lose something. The minimum worth holding to.
Cloud sync keeps the same files visible across devices. Delete something by mistake or let a file corrupt, and that state propagates to the other devices too. A backup is a backup only if it can return you to an earlier point in time. Using a sync service does not mean you have backups.
Keep three copies, on at least two different kinds of storage, with one of them physically elsewhere. Fire or theft takes every copy in the same location together. For an individual, the original plus an external drive plus cloud storage is generally enough.
Backups that rely on remembering are the first thing dropped when you are busy. Turning on the automatic backup built into your operating system or cloud service, so it runs on a schedule, is what lasts. Backups to an external drive can usually be set to run automatically when it is connected.
It is not rare for backup files to be corrupt, or for a misconfiguration to mean nothing was ever saved. Finding out on the day you need it is too late. Occasionally restoring a few files and checking they open is the only reliable verification.
Trying to back up everything often ends in backing up nothing. Start with what cannot be recreated: photos and video, documents you wrote yourself, certificates and recovery codes. Installers and anything you could download again rank lower.
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