So why does only one partner get a ring? Why, typically, the female one? In same-sex marriages, how do partners determine who gets the ring? Flip a coin? Damn.
The answer to all your questions is: it depends on the partners and what they agree to. But on the most part, it is driven by cultural momentum - the same reason men's formal clothing includes a tie. One could equally argue "One does not need to wear a tie to go to work", but there are people who love ties and how they look wearing them.
