To see what causes a change in experiment. If they kept everything the same what would be the point of experiment? One thing has to be different to test and get a result ! They have to see how that one variable changes the experiment. If they change so many things then the experiment will be messed up. They have to focus on that one variable during experiment to get the proper result. Hope this helps you. Think about it if you want to know how antibiotic effects bodies. You choose two people and you give antibiotic to one. And you don't give any other drug to the people you are testing. Because you truly want to know the effect of antibiotic alone.
