This will change the owner of the reg keys to Administrators and give admins & system full control. This will not remove any current permissions, it only adds to them. This tool is also a part of Windows Repair (All In One).
What will this fix? A lot of things actually. Here are a few examples.
You try to install some Windows updates but they fail. If the updates don't have permission to certain keys the updates will fail.
Software wont work. Same as windows updates, if a program cant access a key it needs it will fail.
Registering files gives you an error. When trying to register some system files you get an error that it failed. This is because the reg keys the file needs arent accessible. By fixing the permissions you can again register the files.