CCleaner is not the best system junk files and registry cleaning tool, but on balance it has not bad cleaning effect and less negative impacts to system stability. So it is a matter of course that CCleaner has become one of the must-have tools for PC users.
It is necessary to do defragmentation to hard drive at fixed period, in order to maintain its read/write speed. Windows has built in an "Optimize Drives" already at an earlier time, but its defragment speed and effect are really difficult to satisfy users. So many more powerful third-party disk defragmenters get their markets.