Since the registry is literally the heart of your Windows® configuration (see The Windows® Registry) optimizing it is a particularly critical process. For example, entries that are actually needed may seem to be orphaned. In other cases an application may behave in a completely unexpected way, if a genuinely redundant entry referring to a file or other item that does not exist is removed from the registry.
Registry Optimizer always creates backups of all deleted registry entries so that you can restore them, if necessary. See Backups for further information. Using the backup is playing safe when using this powerful tool.