They secrete milk after pregnancy, and tend to be very sexually attractive to men. Men find women's boobs attractive for biological and evolutionary reasons, they signal to men that the woman attached to them is nutritionally healthy, youthful, and able to have a baby. So, boobs may be one of the main factors of sexual attraction between men and women that leads to reproduction and the continued survival of humans.
Playing with their boobs during sex can be arousing for women.
Because of their attractiveness, boobs are often times used by women to get men's attention, or to get them to do something. Men have been known to go to extreme lengths just for a woman's boobs.
Guy 1: Women's boobs ensure that humans continue to survive and thrive.
Guy 2: No kidding, the first time Emma showed me her boobs I couldn't stop smiling for a whole week.