If you do a bit of a research you realize that sound is a tricky thing. Just because it's pointing to the side it doesn't mean the sound goes in that direction only. For that similar reason most headset mikes - be it a regular mike, or a bluetooth mike - are not being placed in front of a person's mouth, but to the side - that's because it's out of the way while the sound degradation due to the side placement is minimal.
Also, check the car horns - none of them are pointed forward - so those designers had something in mind. As a matter of fact, if pointed forward, any horn will collect dust and debris and may stop performing over time. Also, the horns are placed in such a way that any moisture accumulations could just drip away - another important consideration.
Also, I would and did use the horn to alert a car behind me not to get too close - which is especially dangerous in stop-and-go traffic situations. But my best use of the horn is to wake up a left lane driver doing 50 in the left lane and blocking the traffic - that's where the placement of my horn adds a bit of extra sound.
