Answer:
It permits you to test whether your independent variable has an effect on the dependent variable.
Explanation:
If I am understanding correctly, you are asking why controlling the outside factors affects the results. When you run an experiment, you want to just focus on a small number of independent variables--that of which you are manipulating--and their effect on the dependent variable.
By controlling for other outside factors, you are ensuring that you are only testing that of the independent variables you used in your hypothesis. Other outside factors, as they are not the independent variables, could change the resulting effects--skewing the data.
Human error is inevitable and you cannot 100% control all outside factors (nor do you want to, especially in correlational studies). You can do what you are able and list that of other factors that you could not/did not control that could have impacted your data in your report.
Hope this helps!
