Ahh thanks guys. I was meant to ask, what's the purpose of the experimental group not control
There have been some good answers here already but for further clarification:
The purpose of an experiment is to investigate the relationship between variables.
So if you wanted to test if
exposure to sunlight (independent variable) increases
the height of gum trees you would test how high gum trees grow when 1. Gum trees are exposed to light and when 2. Gum trees are not exposed to light.
If you only had gum trees in situation one or only in situation 2 the results would be meaningless as you would have no idea what the impact of the situation was (as there is nothing to compare it to).
However, if you have gum trees in both situations you can compare them and this draw conclusions about the effect of sunlight.
The way that I have phrased it 1. is the experimental group and 2. Is the control group. However, I could have asked if "non-exposure to sunlight decreases gum tree height" in which case 1. would be the control and 2. would be the experimental group.
Hopefully this demonstrates to you that the experimental and control group both have the same role: to act as a comparison against the other. Neither has meaning without the other.