Why is it absolutely necessary to control the fission reaction in nuclear power plants?

A reaction that splits the nucleus of an atom into smaller parts, releasing a massive amount of energy is known as fission reaction. While an uncontrolled fission reaction is used in nuclear bombs, a controlled fission reaction can be used for generating electricity, to operate a ship and so on. Fission generates 2, 3 or more free neutrons, which are capable of inducing further fission and so on, thus resulting in a chain reaction. This can be initially controlled, leading to further fission, generation of neurons and continuous increase in the amount of energy liberated.