Fitness trackers have many sensors that work together to give you a well rounded look into your workout regiment. Accelerometers measure your steps and help to track your distance. It uses this data along with your personal information, like height and weight, to calculate how many calories you burn during your workout.
Your heart rate is also another important aspect of your workout. Keeping your heart rate above a certain beat will ensure your muscles are working at their maximum and burning through your fat. Most of the waterproof fitness trackers these days will keep track of your heart rate without the need for a strap or additional hardware. All of this would be mute if the fitness tracker didn’t have some waterproofing as well.