To tempt, titillate and transform. It gives a woman something to pose, sit or stand around in as a precursor to sex (or to show off a bit and then withhold it, their prerogative) when just being naked may make them feel innapropriate, or embarrased.
Its a cultural thing too. Cultures where the showing of any amount of naked female flesh is forbidden do not necessarily need such a tool as the mere act of undressing even the outermost garment will be tempting enough to drive the sexual expectation and appetite of their partner up. For example, devout Muslim couples get married without the men needing to know how big the women can make their cleavage appear or how good they look in a basque. In Western society, where women are free to wear crop tops, shorts the size of belts, high heels and bikinis, the amount of flesh on show all the time has reached a higher level and so the woman is looking for something to augment her assets and excite her partner while feeling sexy and beautiful at the same time.