When I use my horn:
1. Test - At least make sure your horn works once in a while
2. if you think you're about to be in an collision so you can tell the Coast Guard you made every reasonable attempt to signal the other vessel prior to the colllision.
3. Fog conditions
4. 4th of July
5. When the ladies on the boat moored in front of you, decide to unfasten their bikini tops to avoid those nasty tan lines*while sunbathing!**