Like I said, it doesn't actually help.
The problem is, basically, that the Vios line-up uses a single core 2/3rd width radiator with a single fan. This makes for faster warm-up and better economy for most consumers, but in traffic, it heatsoaks pretty terribly. And taxi drivers here spend a lot of time idling at a stop while waiting for passengers.
Proper preventive maintenance helps keep the cooling system adequate, but the words "proper" and "adequate" hardly ever come together for some taxi operators. The cheap solution? Instead of wiring in extra fans or installing a higher capacity radiator, drivers simply prop open their hoods while driving around the city. The effect is mostly psychological, but then, when in Rome...