In the ketubah there is a clause ''I will work.'' But in any document, if one writes ''I will do such and such a thing'' it is קניין דברים acquisition of words which has no validity.  So why is it in the ketubah-- because of the obligation that exists anyway. Even though the obligation to provide for a wife is an argument if it is from the Torah or the words of the scribes, still there is no doubt that there is such an obligation which amounts to about two kilograms of flour every week.