Why scientists should use controls in their experiments?

Because If they don't, the scientists won't know what the difference is in.

Example:

You have two glasses of water. One is labeled control.The other is labeled salt. The one labeled controlhas nothing added to it. The one labeled salt has salt added to it. Is there a difference between the two glasses?

If an experiment has a control in it, that means the scientist needs to know if there is a difference in the experiment.