My father told me the reason some years ago. As a rough sketch, it was because decades ago many highways used to be 'single lane', so if you had a truck in front of you, you needed to overtake it as you wouldn't want to be stuck tailing it the entire length of the road.

He said that above the "OK" there used to be a bulb. So if someone wanted to overtake the truck, they were required to sound their vehicle's horn to signal their intent/desire to do so, that's what the "Horn Please" was for. If all was clear, the truck driver used to turn on the bulb to tell that it was "OK" to do so.

Apparently, with time, the bulb went into oblivion (perhaps due to becoming redundant because of the widening of the roads) but the words remain.