Variables change with each experiment, ie amount of water a plant receives would alter growth rate. A control group however demonstrates reactions with all variables being controlled. Let’s say you’re testing whether certain brands of coffee make plants grow better. Your experiment would have plants receiving the same amount of different brands of coffee, with the control group receiving water. This would show whether the coffee made the plants grow more, and which made it grow less. You would compare the results from the experiment to the results from the control.