Unlike registry cleaners, registry defragmentation
typically consists of performing a physical defragmentation and compacting of the Windows registry
in order to remove slack space which reduces the registry size. Whether degragmenting the registry will improve performance is debatable but any changes made to the registry after a defrag are lost when the computer is restarted.
What About Defragging the Registry?
What most people don't realize, however, is that your registry is fully loaded into RAM when your system boots, meaning all reads and writes to the registry are done directly to memory, and then loaded to the disk later. So while you're going to gain some slight performance gain by making sure your registry isn't too fragmented, the reality is that your system isn't going to suddenly be blazing fast because you defragged your registry.
IMO using a registry defragmenter is not really necessary but you may get differing opinions since some folks like to use such utilities.
However, some registry cleaners which include degragmentation features, may peform other optimizing actions which claim to speed up your computer by finding and removing orphaned and corrupt registry entries that are responsible for slowing down system performance. There is no statistical evidence
to back such claims. Further, using registry cleaners unnecessarily or incorrectly is dangerous
and could lead to disastrous effects
on your operating system such as preventing it from ever starting again.
Bleeping Computer DOES NOT recommend the use of registry cleaners/optimizers for several reasons.
Why you should not use Registry Cleaners and Optimization Tools
Be sure to read Microsoft's support policy for the use of registry cleaning utilities
in that topic...Microsoft does not support the use of registry cleaners.