Every time I periodically use MacKeeper it easily frees me up 1GB of hard drive space in the same was that the Windows "Disk Cleanup" tool does. If I have the time and energy to use the duplicate files tool I can free up a lot more space. For people with MB Airs with small SSDs and limited space, this could be invaluable.
A lot of it has to do with which files are being "cleaned up". For example, if you delete a cache file to free up space, that cache will automatically be rebuilt and grow again. If you've consumed that space with user files, there is no room for the cache to grow. Multiply that times all such files and you could have an issue with no drive space available. There should be at least 10% free drive space left available, for better performance. Also, if you're using a tool to remove architectures and languages, you can create serious problems if you remove the wrong thing. Removing some languages can prevent some apps from working or updating properly, as was the case for years with Microsoft Office for Mac.
Again, these tools appeal, for the most part, to those who aren't as technically advanced, who think they need something to keep their system running well, because that's what they're used to with Windows. Any of these tools in the hands of a neophyte can result in problems. If you don't know enough to manually select and delete what you want, then you don't know enough to properly manage the tools that can do the same thing. That's where users have had problems, letting such tools make decisions for them, without the knowledge to say "Wait! That shouldn't be deleted!"
There are many ways of managing space outside of deleting system files, languages, architectures and cache files. Splitting an iTunes library between internal and external drives, using network drives, etc. are methods that any user can do to maximize space utilization without risking the integrity of their system.
Many choose these apps because they claim to make Macs run faster or with better performance. None of these apps improve speed or performance. They only free up drive space, which has no impact on performance, and there are safer methods than entrusting that job to an app, especially one with a less than stellar reputation.