Impact of Control Groups in Experimental Design

A research team is investigating the impact of a new training regimen on the endurance levels of runners. They design an experiment involving two groups: a control group and an experimental group. The control group follows the traditional training program, while the experimental group engages in the new training regimen for eight weeks. At the end of the experiment, both groups are evaluated on their endurance using a standardized running test. Which aspect of experimental design is primarily addressed by using the control group in this study?