They clean the Windows Registry of invalid entries, supposedly to stabilize or repair your Windows install if it is old and you've uninstalled several programs over the years. Most uninstallers are not very thorough and leave behind invalid registry entries which can clog up your system and cause it to become unstable. Removing invalid links from the registry will hopefully stabilize your OS and ensure your system operates as efficiently as possible. The older Windows 9x versions were more susceptible to the registry becoming bloated as compared to the newer NT based versions of Windows operating systems.
As far as performance gains are concerned, you will probably realize a more noticeable improvement from running a stand alone defragmention program such as Perfect Disk, O&O Defrag, or Diskkeeper. However, some folks also prefer to keep their registry as clean as possible, thus the need for registry cleaners. Some programs also compact and optimize the registry, so depending on your needs the aforementioned features can also be found with some software programs whose primary function is to clean the registry.
