great question, even though some wouldn't need any other incentive. But most commonly, to reward the risk they are taking?

Women offer intercourse to a man over the longer term to encourage emotional bonding - keep a man tied into a relationship with them. Women accept that men's interest is sexual and that if a man does not get the sex he needs or wants then he is likely to go elsewhere. In the short-term women hope for lifestyle rewards or just a free dinner. Some women are much more choosy than others. It's a matter of personality as much as moral standards and has nothing to do with responsiveness. No woman is responsive with a partner and most women never orgasm throughout their entire lives. That is clear from women's ignorance over sexual response.