Why do experiments include control groups?

So there is something to compare the changed part of the experiment too. The reason why I created a control group for if My plants grew faster in the dark or the light is to compare the growth speed if there was any. I put one on my porch outside where it was under direct sunlight. I put another on my desk where it got light only on certain parts of the day and I put the last one in my closet where it hadn't received any sunlight. in this case my plant on my desk would be my control group.
Hope this helps :D

Control groups are used as a source of comparison in the experiment, so when you are testing the experimental group, you can see how the results are different than the control group. It makes the results more accurate.