why do scientists use control groups in experiments

Scientists use control groups so that they have something to compare or contrast their independent variable to. Without a control group, they wouldn't have anything to prove i
f their experiment had any important change. For example, a scientist is trying to prove that the bugs in his garden is affecting his flowers.

His control group would be a set-up without the bugs. He would have another set-up WITH bugs. After some time, he would compare the two. If he didn't have a control group, how would he know that the bugs affected his flowers at all?

Maybe the flowers were changing due to another factor, but how would you know? Therefore, scientists conducting research will pick some group of subjects that are as close as possible to the rest of the ones in the experiment, with the sole difference being that they are not getting the treatment.

In our example, the scientists would take two groups with the same age, health status, environment, and so on, trying to match them up as closely as possible.

Then the scientists would give both groups a medication similar in appearance, except that one medication would be the new medication being tested and the other medication would be a dummy that does nothing.

The result of using control groups is that the scientist can isolate the effect of the treatment, and avoid the problem of other variables affecting the result of the experiment.