The only difference is that no experimental variables are changed. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. Experiments are often done many times to guarantee that what you observe is reproducible, or to obtain an average result.
Reproducibility is a crucial requirement.
Without it you cannot trust your results. Reproducible experiments reduce the chance that you have made an experimental error, or observed a random effect during one particular experimental run. Some Guidelines for Experimental Procedures Select only one thing to change in each experiment. Things that can be changed are called variables. Change something that will help you answer your questions.