Now that I've spent a few days as a road user in Cambodia, I understand why everyone honks their horn at everyone else all the time.
As a cyclist, I found myself at the bottom of the pecking order. It was my job to get out of everyone else's way. However, it's difficult to look in all directions at once, particularly without mirrors.
Enter the honking vehicle horn. By the simple expedient of nearly continuous use of the horn, you can warn other vehicles of your approach. It's like a foghorn. You even warn them what kind of vehicle you are, how urgent your passage is, and (obviously) where you are.
Unlike in the States, nothing obnoxious is meant by a vehicle horn in normal use (as opposed to the continuous, panic, hold-down-the-horn-button-until-impact horn), just a 'Hey, I'm here, just letting you know'. Sometimes, if you haven't obviously reacted, it's followed by a, 'Yo, wake up, there's a box truck coming up behind you!' kind of horn, but normally people react to their first notice and everyone continues on their way.
I got so used to having vehicles honk to announce their presence I started relying on their doing so. Surprisingly, this worked well. I'd still look in all directions possible, but it's obviously impossible to cover everything all the time, particularly in Third World traffic.
The taxi driver who took me on the three and a half hour drive from Siem Reap to Poipet would cruise along with a finger on the horn, giving quick taps of the horn as he approached clumps of cyclists. They'd move smartly to the sides of the road and he'd cruise on through without touching the brakes. Everyone did the right thing and no one got hurt.
There are clearly better and safer ways to organize traffic, but in this as in many other parts of life in the Third World, you make do with what you have.