Its for the horn, and i reckon the bracket is upside down too. Ive usually seen the horn mounted under the CDI.
All '80 onwards models should have had a horn, would not have got ADR approved without. If yours didnt have it from the show room, it would appear that someone forgot to put it on when the following the assembly manual when uncrating the bike....Actually no, just checked the assembly manual and the horn was already fitted to the crated bike back at japan. Some of the 79 models had them too.