The love between a man and a woman is special. That is, if none of them is already attached to another other than the person he or she truly loves. Hence, such relationships end up in marriages. It is designed that way since the creation of the first man and woman, Adam and Eve.
With so many people around the world now, though, it is but normal to tell the world that this special woman is yours or you have agreed to be the special woman of a specific man. Declaring your love to each other to the public is often manifested with one of those special engagement rings. It is as if to say, “hey, we are for each other.”