Not at all, actually. Human breasts are so prominent because we store fat there and most mammals do not. The average mammalian mammary is mostly unnoticeable unless they are lactating. Humans are freaks.
"Humans store fat there" is the how, not the why. There are several theories why human females have breasts, every single of them stemming from the fact we're upright (from "easier to breastfeed in that position" to the slightly unflattering yet widely more accepted "mimicking of buttocks").