Scientists use the scientific method to determine if one thing is the cause of something else. It helps them to understand if their hypotheses are true or false.
When using the scientific method, it is important to have a controlled and an independent variable. By adding one manipulated variable into the mix, as opposed to many, one can determine if that variable causes change in the control. It is also important to take copious notes and use accurate measurements to ensure that the experiment may be repeated by other scientists. The experiment is of little use unless it can be replicated under similar conditions.
The scientific method is used so that a hypothesis can be supported. When the hypothesis is supported, it then becomes a theory. One can determine theories to be false with more information and more experiments that use the scientific method. It is through this method that science continues to grow, using observable data that can be measured precisely in experiments that can be replicated by others.