As man learns to make machines that no longer rely on animal or human power, he finds that he has to develop some means to manage and control them. Powerful machines let loosed by themselves will create havoc and destruction.
Control systems have been developed for machines or processes so as to reduce chances of failure and to provide required results.
Basically, there are open loop control systems and close loop control systems.
Open loop systems are those where controller action is not related to final result. This means that there is no feedback to controller to adjust control action. A simple example is when you fill up a tank using a garden hose. As long as tap is opened, water will flow. The height of water in tank will not make tap close.
However, when you see tank becoming full and decide to close tap, you are adding element of feedback to loop. It then becomes a closed loop. But it is a human controlled closed loop.
Closed loop systems use feedback from final result to adjust control action accordingly.
But how would you like to stand in one position, watch process going on and operate valves, or switches according to conditions you want? This is very tiring, isn't it?
Moreover, human beings are prone to fatigue, boredom, and misjudgment. Manual control works very well only if speed of response is very slow, and result is not very critical or important. Human controlled systems can become very unreliable.