Why Is Distilled Water Used As Control? Why Do We Need This Control? What Does Water Contain?

Distilled water is used as a control because its purified. Its devoid of contaminants. Controls are needed so you can see what the results would be of an experiment... Positive controls tell you what the positive result of your unknown would be, and negative controls show you what the negative result of your unknown would be. Without the controls, you wouldn't know what to look for in a positive or negative result.

In most cases distilled water is used as a blank. That blank also serves as a reference solution.
When you are going to measure a sample deviation of the blank indicates the prescence of certain components ( at given wavelengths).  If there were no controls you would not know what to refer to.