Nothing, on the Aus bikes that bracket is original, and not part of the installation for the horn.
I think it is a cover for the neutral switch or sidestand switch.
with respect to the horn installation, have also attached wiring for the banshee system. One thing i found as i retained the stock horn, was that i needed to fit a relay off the existing horn to drive the banshee horn which was supplied direct from the battery..
hope this helps..