Why do the pilots dump fuel?

Going with the question, Firstly, you must know that a plane cannot land with weight more than maximum limit. That may cause many hazards. As I have mentioned in my previous post the plane needs too much fuel to operate. And when landing  the plane must be lower than the maximum weight. Actually, the pilots dump fuel in very exceptional case. They actually dump the fuels in case to reduce the weight to make it up to the maximum limit. For an example if any of passengers are in extreme condition and need treatment then landing on the airport. Then to get the required weight the fuel is dumped.