For those who have long been working out, the resting rate of their heart is a number familiar to them. They know how much effort in the gym it will take to get a faster heart rate, so they know how much exercise they will need for a good workout. Their body has already been pushed past its normal resting limits, and their goal is to make those limits expand up and down for better fitness.
Sitting does little to help a body when it comes to movement, and those who take the time to workout at a gym have found their lack of movement can compromise their ability to exercise at first. Their resting heart rate is often much higher than it should be for their age and weight, and it takes very little movement before it rises quickly. Working out will help their body tolerate a higher rate when moving, and it will make their body work less and have a lower rate when resting.