Heh, that link is pretty cool.
The main purpose of a mix bus compressor is to take up space on Gearslutz forums. It works really well.
Seriously, just do a search, you'll find about 30 threads about them just in the last month or two.
Basically, it's a stereo compressor on your main output. Some use it to tame peaks, raise apparent loudness, and add "glue" to a mix.
You can also do a search on this forum for "glue," or just go to http://www.elmers.com