Flexi Says: Controlled variables are important to identify in experiments because they can change the outcome of an experiment in a way that makes it invalid. Control variables must be kept constant to prevent them from influencing the effect of the independent variable on the dependent variable.
For example, if you were to measure the effect that different amounts of fertilizer has on plant growth, the independent variable would be the amount of fertilizer used (the changing factor of the experiment). The dependent variables would be the growth in height and/or mass of the plant (the factors that are influenced in the experiment). The controlled variables include the type of plant, the type of fertilizer, the amount of sunlight the plant gets, the size of the pots you use. The controlled variables are controlled by you, otherwise they would influence the dependent variable.