The registry never needs cleaning.
That is simply nonsense. It is completely and utterly wrong.
In an ideal world, maybe. But in real life when you upgrade things or uninstall things the upgrade or uninstall software might be crap in which case it can leave stuff behind in the registry which can stop other things working.
Going through and finding the dangling entries and deleting them by hand is one approach, but using a tool which claims to be able to do this automatically is another. Manually it is fairly straightforward to find keys or values that point directlyto files you've uninstalled, but few people want to chase through the indirect chains of GUID references by hand, so the tools have their place.