Why there is a need to control variables not of interest in the study?

Because you want to make sure the variable of interest (the independent variable) is what is actually causing the outcome (the dependent variable) and not some other outside influence that is present at the time of the experiment.





For example, you want to see if exposure to UV radiation causes skin cancer. If you do a study on UV exposure and see a correlation between the amount of UV exposure and skin cancer, but don't control for other possible sources of skin cancer, you don't know if the cancer is caused by UV radiation or some other factor such as heredity, chemical exposure, or some other source of radiation.