What does the GM spark control do?

[mdcptman]

Ok, i found this on ebay

http://www.ebay.com/itm/90-gmc-chevy...f6fa9e&vxp=mtr

This is kinda a stupid question, but what does it do? Is it in addition to, or replace the module in the GM distributor? I am wondering, because if it controls the spark separate from the ECU, can it be added to control spark on a fuel only TBI conversion. OR, Say I convert to TBI, and opt to use the Duraspark distributor with vaccum and mechanical advance disabled, can this module be connected to the ECU instead of connecting the GM module outside of the distributor as I have seen others do.
I hope this question makes sense. thanks in advance for the input!