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What "registry optimizers" mainly do is clean out obsolete or unused registry entries left behind by earlier versions of programs you've installed on your computer, or from programs you've uninstalled. They seldom do any damage, but also rarely do anything meaningfully good. It's mainly trivial "housecleaning" that won't appreciably improve your computer's performance.
While they're not exactly a scam, I don't see any reason to pay for a "registry cleaner". If you're really intent on trying one, some antivirus software vendors (like Norton) include one as an incidental extra in some of their packages, and there are lots of free ones available on-line from various sources.