This is so that every muscle in your body can be targeted. Also, it provides variation so that you don't get bored and give up. If you don't like exercise, revert to what your ancestors did instead and walk everywhere, move constantly and do plenty of physical work at least once a day, such as chopping wood, gardening, carry loads or cleaning your house vigorously.
Circuit training is a fun way to focus on strength and aerobic exercise and is great especially if you love some variety and a faster pace of working out. It’s worth noting that circuit training doesn’t offer the same level of conditioning as strength and aerobic exercise does on their own. Specifically, if your fitness goal is to be strong, circuit training is not the best way to reach that goal. You can do circuit training in groups or classes as well as alone. This article offers an easy way of creating your own circuit routines.
In summary, if you're only interested in a basic understanding of HIT methodology and where much of it originated I would suggest starting with a far less technical book. I suggest starting with the last published edition of Ellington Darden's "The Nautilus Book" and perhaps "Total Fitness: The Nautilus Way". If you like what you read and want to dig a little deeper into the evolution of HIT read Darden's more recent book, "The New High Intensity Training: The Best Muscle-Building System You've Never Tried". If the gears in your head are in high gear after that and you really want to get DEEP into what evolved from the original Nautilus protocol _then_ you go for "Superslow" or preferably "The Renaissance of Exercise: A Vitruvian Adventure Volume 1". When your grasp of all the aforementioned material is truly solid then move on to Doug McGuff's writing. McGuff's ideas do not surpass or supplant Hutchins' but rather sharpen the points with brilliant thoughts and clinical observations from a medical physician's perspective. Doug McGuff, MD published his "Ultimate Exercise: Bulletin #1" in the late 90's and later updated that with "Body by Science: A Research Based Program to Get the Results You Want in 12 Minutes a Week", both of which are hugely valuable contributions to the literature on HIT methodology and philosophy. His article about "Stoicism in Training" is critical reading.
Exercise is any bodily activity that enhances or maintains physical fitness and overall health and wellness. It is performed for various reasons, including increasing growth and development, preventing aging, strengthening muscles and the cardiovascular system, honing athletic skills, weight loss or maintenance, and also for enjoyment. Many individuals choose to exercise outdoors where they can congregate in groups, socialize, and enhance well-being.
Yoga is appropriate for individuals of all ages and fitness levels. Those who are seeking a workout that increases their mind, body and spiritual awareness will find yoga to be a good fit for them. Those who wish to increase their fitness level at a slower pace may find that yoga is perfect for them, although the physical intensity is just as high as in many other exercise methods. Yoga exercises utilize a variety of muscle groups at the same time, so an entire body workout is often achieved during each class. Yoga is effective in toning muscles without creating a bulky look.
Thus, little is known about the effects of monitored vigorous exercise in elderly people. While significant benefits for basic motor tasks (such as balance and gait) can be achieved through different kinds of physical activity (i.e., stretching exercises, treadmill, Pilates, and strength and balance training), no conclusive relationship has been proven between its intensity and such improvements. Recently, Pau et al.  reported that spatiotemporal gait parameters and sit-to-stand performance significantly improve through vigorous (but not light) exercises, thus suggesting that higher levels of intensity might be more suitable in generally improving static and dynamic daily motor tasks.