Why does the speaker refer to swimming as one of the most beneficial forms of general exercise?

Swimming is a popular competitive sport, and many age groups, interscholastic, and intercollegiate meets are held each year. Each individual swimming race is classified according to the stroke required and the distance covered.
    Swimming is known not only as a means of survival or saving lives in emergencies, but also as a valuable means of physical improvement and as one of the most beneficial forms of general exercise. No other form of exercise uses so many muscles in the body so fully. Swimming’s popularity has been helped by the fact that many indoor and outdoor swimming pools have been constructed for public use worldwide.
    Now that you’ve listened to me talk about all the rewarding aspects of swimming, I’d like to talk to you about some of the aspects necessary to become a dedicated swimmer. Competitive swimming is one of the most demanding of all sports. Since all of you today are interested in joining the swimming team, you might be wondering where your training here can lead. That’s a good question and not a very easy one to answer. There’s high dropout rate a long competitive swimmers. If we compare swimming to a sport like running, you will note that runners will often continue to train and compete in races after the age of thirty, but most swimmers will stop competing after they graduate from college. There are probably two major reasons for this. The first is financial. Unlike runners, tennis players, or golfers, swimmers cannot make much money in their sport. Swimmers compete for the pleasure and the glory of winning, not for the monetary rewards. The athletic scholarships that colleges award to the top swimmers are probably the culminating financial rewards that swimmers receive for their swimming abilities. The second reason for the high dropout rate is a physical one. Most top competitive swimmers undergo hard workouts four hours a day, eleven months out of the year. Swimmers pay a heavy price in developing the physical and mental discipline necessary for swimming excellence. For most swimmers, this type of schedule is too demanding in terms of time and too draining in terms of physical energy. So knowing all this, if you are still enthusiastic about competitive swimming, I’d like to hand out this schedule of tryouts...