The control is an important aspect of an experiment because it establishes the baseline that the experiment's subjects are compared to. Without a control, researchers would not have anything to compare the experiment's results to. Controls are also helpful because they permit scientists to identify possible mistakes or errors in the experiment if the control's results are not what they anticipated.
If an experiment was comparing the effects of different fertilizers on plant growth, the plants that received no fertilizer are the control group. The other groups of plants that were treated with different kinds of fertilizer have their growth compared to that of the control growth to determine what fertilizers are effective.