Over the centuries females attempting to increase their mate choice have dressed to exploit this shape (corsets, bustles and wonder bras). If ancestral males had not shown a preference for the mutation producing symmetrical, plump bosoms, modern women’s chests would resemble the flat thoraxes of the other apes.
This is an excellent example of bad science (here, evolutionary psychology), of the androcentric kind (as often with this proto-science). The human female breasts are a sexual attraction and this fact only, supposedly, can explain it is so large (in general), even when it is not useful, etc.
In fact, it is the retention of fat in the buttocks, breasts and brain, as David Horrobin witted, that distinguishes humans from others.