Backup and sync-between-machines are not the same thing!
Office 365 materials describe OneDriveBackup as backing up your content. But that's not what it does, if you have multiple machines. Instead, it merges all of them into one synchronized set of folders, which is not at all the same thing.
I want to back up my laptop, and I want to back up my desktop. That doesn't mean that I want to merge their content.
Having a merged Desktop/Documents/et cetera is a great option, but let me keep the different machines separate if I want to.