In an experiment, a control is used to establish a baseline to compare the results of the main experiment to. Having a control group rules out any environmental variables, such as temperature and humidity, from affecting the conclusion of the experiment.
Most experiments aim to prove if a method or material is better or worse than an existing method or material. The two experiments being compared to each other can't be done at different times or different locations, or there is a significant risk of there being confounding variables. Most experiments have both an experimental group testing the new variable and a control group that employs the normal settings running simultaneously to return valid results.