From my perspective, it's a legit question, with legit answers:
1. Trailering, and you see a bad situation developing. The louder your horns, the earlier you can effectively get another drivers attention and maybe, just maybe, the situation doesn't deteriorate further. You have more time to brake, they have more time to get the *&^% out of your way.
2. Distracted drivers, who somehow feel you're not honking at them. Louder = more change to make them peel their eyes off the phone/nav/lunch or whatever.
3. Mega-watt bass thumpin' stereos. You need to be able to pierce thru all that noise inside the car. If you can hear it that loudly in YOUR vehicle, imagine what they're hearing inside theirs.
These are just off the cuff at the moment, sure others will come up with more.