Yevamot 89: Upholding Rabbinic Law over Torah Law

Why did the rabbis decide that women need a ketubah in marriage to begin with? And when is the absence of a ketubah helpful? Also, a new understanding of our mishnah, specifically with regard to a child born being a mamzer - in the context of terumah, impure produce, and improper designation of sanctity. Plus, to what extent is there a human component even in the determination or designation of mamzerim? And under what circumstances do those rabbinic stipulations or decrees override (and undermine) the Torah edicts?