Well, Amazon might fail as a company at some point and then all your data will be gone. Theoretically.

Much more likely, though, is that you, or some sysadmin at your company, or even some user will accidentally hit the "delete" button on something important, and then without a backup, you can't get it back. Which is honestly the thing that people usually need their backups for, anyway. This is what most "data loss incidents" are: people just messing up and deleting things they shouldn't have. Wetware is much more prone to failure than hardware, after all.

There are ways to protect against delete or overwrite, for example by using versions.

Does that work when the sysadmin fat fingers the deletion of a bucket or account?

> Why would you want backups for S3?

In case something goes wrong. ie your account has a problem unexpectedly, or if they do indeed have a data loss/corruption problem, etc.