What Does Registry Cleaner Mean?
A registry cleaner is a third-party Windows OS utility software whose sole purpose is to clean up the Windows registry and keep the system error-free and fast by removing redundant items like the registry files of already uninstalled software or shortcuts that have since been moved.
This type of utility exists because older versions of the Windows OS tend to slow down during boot up and exhibit errors when the registry becomes too big and cluttered.
Techopedia Explains Registry Cleaner
The need for registry cleaners came about because older versions of the Windows operating system tended to slow down because of the size and complexity that the registry database after a while of use. Manual cleanup of invalid entries, missing references or broken and invalid link registry entries becomes impractical because of the sheer number of entries, so finding the entry in question would be tedious, therefore the need for automation becomes apparent.
This is not so much of an issue in NT-based OSs like Vista and XP because of a different registry structure and improved indexing and memory management compared to Windows 9x versions. Microsoft has never expressed the need for a registry cleaner or else it would have made its own. Additionally, malware and spyware are often associated with these types of freeware, so they are often not recommended.