WHen you add more resistance in your cardio machine workouts, what is the purpose of it and what are the benefits?

When I used to workout at planet fitness I would see an option to add more resistance and I would choose the maximum amount on the stair master and elliptical. What is it and what affects does it give me for workout?

WHen you add more resistance in your cardio machine workouts, what is the purpose of it and what are the benefits?
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