Why is a control group important in an experiment?

Without a control group, what would you compare your results to? How would you know if the treatments had any effect? Without a control group, there is no basis for knowing if a particular result is due to the variable being tested or to some other factor.

The control group gives a set where the expected outcome is known. So when a variable is changed and if the outcome is the same as the control group, then we know to try a different variable.