The Duplicate Finder is meant for removing duplicate files from your file collections, such as collection of MP3 music files or PDF documents. There really isn't any rule of a thumb of what files you can just delete if you run the Duplicate Finder to your entire system or system drive(s).
And there's really no point in doing that either, modern computers have so huge hard disks there is really not much need for any disk cleanup utilities nowdays. Except that you might want to delete some specific kind of files (e.g. temp files), but you can do that with the File Cleaner.
