What are the importance of a control experiment?

A control show that under the normal conditions what will happen.

Example:

Testing effect of light filtering on growth of plant

Tested plant:

8 CM under light with 2 layers of cyan colored plastic.

Control:

8 CM under light with 2 layers of clear colored plastic.

we still need the plastic to keep conditions the same except for the thing we are testing which is not the plastic, but the color's effect.