Marching is done at 120 steps per minute. Running is at 180 steps per minute. There are a few reasons. When the entire unit is bopping to the same sheet of music, everyone stays in step. That looks good, and you don't have people tripping over someone else's feet.
Breathing is another reason.
If you are forced to exhale by singing, you will also be forced to inhale when you are not singing. Motivation is the third reason. Calls that are humorous or have a plot can make soldiers stop thinking about the cold and rain, or how much their feet hurt. Nobody really knows. A recurring character in cadence calls is a person named Jodie.