The reason you use a true experimental design is that you want to have the best chance you can to determine causality. You want to understand causality as it gives you the best information necessary to predict and/or control psychological outcomes.

Thus, you must use manipulations of the IVs to see if your hypotheses are correct. You must be careful to avoid confounds.

Without experimental research, it is difficult to really make the case that TV is related to violence or smoking causes cancer. Look how folks argue about these topics. However, a good experimental design can convince you.