The third perek of Masesekhet Kiddushin begins on our daf. The first Mishna presents us with a case of someone who was appointed as an agent by his friend to marry a certain woman. The Mishna teaches that if the man goes to that woman and offers her kesef kiddushin for himself, she will be married to him and not to the first man.
The question that is raised by the rishonim is why the Mishna needs to teach us this halakha. It would appear to be obvious that if the woman accepted the money and agreed to marry him, she is married to him and not to another! Several suggestions are raised to explain this case.
Tosafot Ri”d suggests that we may have thought that the messenger was not serious in his proposal since we do not suspect that a Jewish person would behave in such an uncouth manner (see Tzefaniah 3:13). The Mishna therefore needs to tell us that such a marriage is a serious one.
The Ramban suggests that we may have thought that this case should be ruled kiddushei ta’ut – a mistaken marriage – since had the woman known that there was another man who wanted to marry her, perhaps she would not have agreed.
According to Tosafot, the case may be where the messenger first told her that he was representing someone else, but then said, “Harei at mekudeshet li – behold you are betrothed to me.” The Mishna teaches that we do not assume that he was still acting as an agent if he says that.
Another approach argues that on occasion the rabbinic sages rule that someone who behaves improperly loses his right by law – and that perhaps the Sages would have negated the marriage retroactively in response to his inappropriate behavior.