I understand that registry cleaners simply sort out or find the items in the registry which have no possible connection with or link to any other program, folder, file or item on the computer and consequently one assumes that such an item is simply a registry entry which is doing nothing and cannot do anything as it has no connections to follow. This item is then taking up space in the registry and is another extra item for programs, etc to check when searching, thus fractionally slowing down the operation.
These unnecessary items are mostly left behind when programs, etc. are removed from the computer as CCleaner invariably explains.
I've used CCleaner for about 10 years and have never had it remove anything which was required later. Just to be on the safe side CC recommends and offers to make a copy of all that it removes so that they can always be replaced back in the registry if needed.
CC will also protect and save those important Cookies for you.
I find CC very useful and will continue to use it