What is the purpose of using a control in scientific experiments?

The experimental control provides a base-line result or set of results, from which you can compare the variables' effects against. It's designed to minimize the effects of variables (other than the single independent variable). Control groups are often included in medical or psychological experiments so that the results of an experiment are considered reliable and trustworthy.

Example:

A mystery-drug cure is being tested. One group of patients is given the drug and the other group is not. The group without the drug will be subject to all the same conditions that the other group are under, thereby seeking to eliminate any unforeseen effecting environmental factors. This makes it possible to compare, and therefore measure, the impact any drug would have.