What is the intended purpose of CCleaner? Is it to get rid of unnecessary files to make the computer run better or is it used to hide unlawful activity, like the Evidence Eliminator type programs?

Its intended to clean junk/temp files. It can also erase history, cookies, typed URL's, open/save dialogs, windows run as well as many applications recent file history. Windows logs, Error reporting, recycle bin, memory dumps and more. You can do secure wipes of files (as opposed to deleting them). The only function I see in EE that CC dosent have is clearing the swap/page file.

So is this a good app for getting rid of data to prevent others from retrieving it? Or is this best for getting rid of unnecessary data on a computer hard drive? If I want to securely get rid of data, would another tool be better suited?

CC is a good app for both uses. If you're like the vast majority of us (i.e you don't hold the Armed Forces' nuclear war codes on your pc) then CC is fine for secure deletion. Despite what a great many may think, there is no evidence of data ever being retrieved after one overwrite by anyone. And CC's overwrite is just as good as the next man's. If you do hold secure data, such as commercial or personal data (others' credit card details, bank details etc) then you need to follow a secure data policy which may include encryption, secure pc access, destruction of old media, etc. If you're a crook (not you personally, in general terms) then you're on your own.