Why did the rabbis institute ma-amar? TB Yevamot 52

I can only conjecture why the rabbis instituted ma-amar (מַאֲמָר), levirate betrothal, when the Torah only requires the surviving brother to take the widow into his home and have intimate relations with her. Based on previous dappim and today’s daf TB Yevamot 52, I want to suggest three reasons why they instituted ma-amar. First the text 

“The Sages taught: How is levirate betrothal performed? He gives her money or the equivalent value of money and declares: You are hereby betrothed to me. The Gemara asks: And with a document, how does he betroth her? The Gemara is puzzled by this question: With a document, how does he betroth her? It is as we have said by the halakhot of a regular document of betrothal: If he wrote to her on paper or on earthenware, even though it is not worth a peruta, the words: You are hereby betrothed to me, it is effective. As a document is not effective as a means of betrothal due to its monetary value but rather due to the words it contains, there is no requirement that it be of a minimum value. However, because the halakhot of betrothal by a document were already taught, the Gemara is puzzled as to the nature of this question. Abaye said that this is what the baraita is saying: With regard to the document of a marriage contract for levirate marriage, how is it written? Abaye understands that the question did not refer to the document of betrothal but rather to the marriage contract of a levirate marriage.” (Sefaria.org translation)

We are human beings created in God’s image and must act appropriately is the first reason. Although we shall learn when we study massekhet Kiddushin having intimate relations with a woman is one of the three ways a man can betroth a woman, today’s daf reminds us that the rabbis forbade this method. “As Rav would flog one who betroths a woman by intercourse, despite the fact that betrothal is effective by this method, because he acted in a promiscuous manner.” (Sefaria.org translation) Animals couple up instinctually without betrothal, but human beings must enter the institution of marriage with forethought and intention.

Ma-amar provides the widow some protection is the second reason. She really has no say according to the Torah in her fate. Since ma-amar is akin to betrothal and a woman cannot be betrothed without her consent, she can nix one yavam in favor of another or demand halitza if the match is inappropriate (e.g. she is young and the surviving brother is very old or repulsive)

The institution of ma-amar elevates the status of the widow, yevama, to a wife in the surviving brother’s eyes is my third reason. With this elevated status, he should treat her appropriately as his wife in all aspects and not just as an obligation to be done with as soon as possible.