walk on the balls of his feet

ChandlerのThe Big Sleep を読んでいます。
I finished my cigarette and lit another. The minutes dragged by. Horns tooted and grunted on the boulevard. A big red interurban car grumbled past A traffic light gonged. The blonde leaned on her elbow and cupped a hand over her eyes and stared at me behind it. The partition door opened and the tall bird with the cane slid out. He had another wrapped parcel, the shape of a large book. He went over to the desk and paid money. He left as he had come, walking on the balls of his feet, breathing with his mouth open, giving me a sharp side glance as he passed.
"walking on the balls of his feet" の個所ですが、最初 "walking on the balls" まで読んで、何かのボールの上を歩くことを、特にバランスボールをイメージした、想像したのですが、それは可笑しい。よく読むと後に "of his feet" とあるので、これは 1/29/2014 に取り上げた "ball" の意味だと気が付きました。しかし、何故わざわざこんな表現を使うのか気になります。
ネットで調べると歩き方について次の様な記述を見つけました。
・While many animals walk on the balls of their feet, humans use a heel-first stride.
・Humans, other great apes and bears are among the few animals that step first on the heel when walking, and then roll onto the ball of the foot and toes. Now, a University of Utah study shows the advantage: Compared with heel-first walking, it takes 53 percent more energy to walk on the balls of your feet, and 83 percent more energy to walk on your toes.
・Why Humans Walk 'Flat-Footed'
・Walking on the toes or the ball of the foot, also known as toe walking, is fairly common in children who are just beginning to walk.
・tiptoe (Oxford English Dictionary): Walk quietly and carefully with one's heels raised and one's weight on the balls of the feet.
歩くときに足のどの部分が先に地面に着くのかの話ですが、人は普通踵から着くとは余り意識していませんでした。