Why are some vehicles tires filled with water? What is the reason behind it. Is it done for all the vehicles or only heavy vehicles?
This is common practice in the agricultural industry ( I worked as a agricultural contract driver) as it is an easy and cheap way of adding weight to the tractor when needed.
It works as the relatively low rotating speed of the wheels ( tends to be the rear mostly) means the water stays in the bottom part of the wheel.
Edit: the extra weight can also be to help balance the tractor if you are using a front loader for example.