As you may have already read, CleanMyMac Classic scans and cleans out caches, logs, language files, universal binaries, system junk, and app leftovers. It is completely safe to delete these kinds of files and located below is the explanation for what happens after cleaning certain select files.
Caches: When you clean up your caches, you force the application to automatically generate new, lightweight caches on the next application launch, and only if the app feels it’s necessary to generate them.
Logs: If you clean log files, your applications will create these files once again. However, they will be free of outdated data.
Language Files: Mac OS X has built-in support for dozens of languages you will never need. This was designed and implemented for the off-chance that you would need them and could easily switch between them. After deleting these localizations (language files), your Mac OS X will have only the languages you use.
Universal Binaries: It is completely safe to have apps with only the architecture that supports your Mac system. This means that you can safely clean universal binaries to free up additional space. Besides, since Lion supports only Intel-based Macs, there is no reason to store a universal binary code for a PowerPC-based system.
System Junk: These files are intended for debugging purposes only and the deletion of them won't affect the performance or stability of your Mac OS X in any way.
Leftovers: When you delete an application, widget, extension, or plugin by moving to the Trash, it usually leaves preference files and other files behind, because they were generated by the app after its installation. These files are completely useless and, therefore, safe for deletion.
In rare cases, the only thing you may need to do after cleaning your Mac is to restart the applications open during your Mac cleaning (or your Mac OS X itself) so that they can automatically begin generating new caches to continue working properly.