CCleaner and other programs like it look for duplicated items in the windows Registry.
It also looks for 'temp files' created by installing programs or uninstalling programs that did not 'clean up after themselves'
Windows' own disk cleanup is generally good enough for most people to perform this task, however CCleaner has an option to make sure all browsing cache and data is really erased. (A moderately experianced computer guru or a really pissed off wife can find out everything you have been browsing to even after you click the IE or Windows native 'wipe / clear history' buttons.)
I have found similar programs VERY useful on my phone, as Droid system makes it a PITA to clear out useless data filling up my phone, and the CCleaner type programs are very effective.
Part of the reason is on a phone you have 16GB of phone storage available, 6+GB is usually already taken up for the operating system. Also pictures dump to the phone storage by default. On modern computers there is ample space and deleting some temp files is not paramount to what can be ran on the computer.