Hi Barrie,
The auto-save (with backup) is designed to help you recover your un-saved work if you have a crash during your session. This is not a versioning tool, it just keeps one copy of the project. If you press save, the backup is cleared out as you have already saved the work.
When used without the backup mode, it is exactly the same as pressing the “Save” button manually every 10 minutes or whatever the frequency you set it to save as.
Regards,
Rohit