Bananas are very rich in carbohydrates, like potatoes, which makes them good for extensive physical labor and anaerobic exercise. Almost all people nowadays burn only about 2000kcal per day. But if you happen to be, for example, an unskilled worker of the early 20th century, who constantly has to shovel coal by hand into a blast furnace, then you could regularly burn 4000 kcal or more per day. The same is true if you bicycle hard hundreds of kilometers 6-10 hours a day.
Bananas then are essential and easily accessible, to provide this kind of energy demand in a healthy fashion.
Since almost all jobs are automated now, such energy demands are rarely ever raised by any kind of job. The banana has become too energy rich, in modern times. Other low-carbohydrate foods, like lettuce, kale and broccoli provide more benefits over the same kind of nutrient types with less excess energy in ratio to fibre and nutrient balance.
That doesn’t mean you shouldn’t eat any bananas. Just not really more than 1-2 a week.
Bananas have, like any other food, no intrinsic superiority concerning bodily organs or parts. That would be folklore and esoteric belief. For example, some people still believe beetroot to be good for “blood”, simply because its red.
Because bananas provide so much energy, its easy to get fat eating bananas. Just like with candy. In fact, bananas are one of the worst and least beneficial fruits in terms of weight loss. Some diet plans even exclude bananas entirely, because they contain so much more energy than any other fruit.