The answer is two-fold. First, some of them are obvious. For example, "Recycle Bin" is literally just the recycling bin that you see on your desktop. Clearing your Recycle Bin with Disk Cleanup is no different than if you manually emptied it. Similarly, "Downloads" is whatever is in your Downloads folder. Whether you want to delete the contents of your Recycle Bin or Downloads folder is entirely up to you.
Second, there are a bunch of file types that Disk Cleanup targets for deletion that are so tiny they don't really amount to much. Deleting these files wouldn't amount to any real gain in storage space, and for that reason, we decided to omit them.