I sorta wrote in tongue-in-cheek. Oh, wait a minute-- no I'll not expand on that.
Anyhow, on a more serious note, if the woman has orgasms, thus enjoying the sex act more than if she didn't, then she would more likely want to engage in sex, which increases the chances of pregnancy, which in the "olden days" would be an advantage to most bands.
In anthropology, we sometimes use the term "cultural selection", namely those items that are selected for that may not have that much directly to do with natural selection but for whatever reason got selected for anyway. Human female breasts, for example, tend to remain full even when not lactating, whereas the ape line's females' the breasts tend to deflate, much like the Patriots' footballs.
Even though the fuller breasts aren't likely to affect survivability, nevertheless they got preferred over the deflated ones.