why is it necessary to create backup and restore after cleaning the registry?

The registry is one of the most important features of Windows. It hold information on literally everything on your computer - all our programs, hardware, files, etc. If something happens that damages the registry, your computer may not boot, or you will lose access to programs and files.

A registry cleaner makes changes to the registry. It deletes stuff out of it and corrects entries in it. When you run a cleaner, there is always the chance that it might decide to change something that it should not and damage the registry. The backup will allow you to "undo" the damage done by a registry cleaner by restoring it to the way it was before the cleaner was run.

It is just a safety feature in case something goes wrong. I would not want to "clean" the registry of my PC without making sure I had a way to undo it if it goes wrong.

The main job of a Registry Cleaner is to remove config data from the Windows Registry that are no longer in use or unwanted by the system. And there are times the cleaner just deletes the keys that may be required.

And a poorly designed registry clean tool may not know for sure whether a key is still being used by the OS or what detrimental effects removing it may have. This has led to examples of running registry cleaner tools causing loss of system instability/functionality.

That's why you should be prepared enough to have a backed up registry in case you are not satisfied with the system performance.