Yevamot 52 - Get, Yibum, Chalitzah
What are the possible permutations of marriages and divorces, and also yibum and chalitzah? If one made a declaration of the intent to marry his deceased brother's wife, but then changed his mind and gave her a Get (divorce document), the Get is not enough. Since the actual Torah law dictates either a yibum or a chalitzah in this case, he needs to also give her a chalitzah.
If one made a statement that he will marry the widow (this is called "maamar," and looks like an engagement), and then cohabited with her - this is the proper way of doing the mitzvah of yibum. Now, that's obvious, isn't it? You might think that since this procedure looks completely like marriage, then people may think that is actually a regular marriage and not a yibum. Therefore, we need to be told that this is the proper order, and that people will understand. Note that in practice today yibum is never done, but rather one performs the chalitzah.
Moreover, one who does not follow the laws of propriety was subject to lashes given by the court. For example, Rav used to give lashes to one who cohabits with his deceased brother's wife without first making a declaration of marriage intent. He also prescribed lashes for one who betroths a woman by cohabitation. Even though this is the only form of engagement actually mentioned in the Torah, the Sages decreed additional, preferred forms: with money (ring) or a document. Some say, Rav only gave lashes in this last case, because it is licentious behavior.
Art: The Marriage Contract by Jan Jozef, the Younger Horemans