PhoneLog tries to prevent you from losing data, by saving changes automatically. It also stores previous versions of the call log as backups so you can view them later and revert back to an old version if you have made changes that you don't want to keep.
A manual backup can be made at any time by you whereas automatic backups are taken at regular intervals and gradually deleted as time passes. In the Preferences you can specify how often the backups should be made and limit how much disk space the complete set of backups should take.
You can choose to revert to an older backup or simply to view the data in a backup so you can retrieve details for individual calls.