I'm not sure that I would use a loud horn in the traffic situation depicted in the OP's video, but I would use it from time to time on neighborhood streets. For example, if you're riding down a street and a car looks like he's ready to back out of a driveway in front of you, or someone pulling out in front of you at an intersection where you have the right of way, a short tap on a loud horn might be appropriate. If I got one, that would be its application, for me anyway.