The rhythm of your heart changes with every single beat. The variability between beats is a good indicator of your overall wellness and adaptability. A heart rate with less variability between beats suggests that the person is older, or suffering declining health. Heart rate variability is a very important fundamental rhythm which reflects the resilience of our hearts battery pack. It's all about energy. If your heart rate variability is low, then you deplete your battery pack more quickly, than someone with higher variability.
Heart Rate Variability predicts disease and lonjevity Researchers can measure your rate rate variability, and guess within about 2 years of your age, IF you are following a healthy aging process. If environmental pollutants, trauma, stress, disease etc have played a factor in your life, then your heart rate may have less variability. Low variability leaves you more prone to disease, faster aging, less fitness etc. Essentially it makes you less adaptable to the stresses of life.