When you're cleaning out your hard drive, deleting just the .app of an application you no longer need is not enough to get rid of it completely. Nearly all applications have additional preference files or plugins that are stored in other, way harder-to-find folders. App Cleaner searches for these files automatically and prompts you to delete these files as well. More or less seamless - just delete the file as you normally would and it does the rest.
Drag and drop option, if you're into that sorta thing.
10/10 for not cluttering up my hard drive needlessly.