These are also regular Macintosh Applications, but they are only used when you download a file. Most files you download will be compressed, or they will be encoded, or both. When Black Night downloads a file, it does an automatic file type check to identify the type of file and any compression or encoding.
Once identified, Black Night will check Internet Config to see if you've specified a post processor to use with that file type (by default most file types and post processors are pre-specified). If you've checked the box, then Black Night will launch the post processor (e.g. Stuffit Expander, JPEG View), and tell it to open the downloaded file.