Well Jim,
think of each of these devices as a remote serial port for the Empeg.

You know how there is all these amazing things we want to do/plug into our Empegs, but we are limited as we only have the inbuilt serial port to connect them all to?

Well this bypasses that limitation, and provided we have a "in car" ethernet connector fitted to our sleds [and the right software], we can have as many serial devices on the Empeg as we want - each one doing one thing:
like:
GPS data stream from the GPS,
OBDII interface data,
External display interface
External buttons or control devices

the list is potentially endless....