Why a woman always become a show piece.?

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You all most have seen that in any commercial advertisement always there is a lady showing her private body parts, whether it is a soap ,shampoo for ladies,or shaving cream for men. What is the concept behind this? If there is an advertisement of condom then we can understand a man and woman need to complete that advertisement, but if there is advertisement of fastest car, what is the necessary of a woman wearing a bikini for that advertisement?