In a pre-test, post-test design why is it important to have a control group?

I need to know why a control group is important or its uses in a randomized pre-test post-test design. I know it's used for comparison and it can account from variations for taking a measure twice, but what else. I don't know if it matters but the participants would be going along with the study under deception. In a pre-test, post-test design why is it important to have a control group?
Having a control group in a pre-test/post-test design helps guard against threats to internal validity. By using a control group, you can minimize the chance that any differences you find are actually due to variables outside your experimental control (e.g., maturation of subjects).





~Dr. B.~