Think of it this way, Tadao. To a certain point, free-swinging boobs are great and actually build up the pectorals, keeping them perky. However, not all women are blessed with fantastic not-too-small-not-too-big boobs. Some of us saw a meteoric rise in cup size after having children, and were big enough already.
Giant boobs that bounce aren't only painful, they can also stretch out over time and eventually get that saggy old-lady look where it looks like the nipples are about in line with the navel. In that case, bras like the one above keep big boobs perkier longer.