According to Rebbi Yishmael, the mishna says one is allowed to prepare for Shabbat on Yom Kippour (day before Shabbat) because the former is holier than the latter. In that case, why is there a need for an eruv tavshilin on Yom tov preceding Shabbat using the same logic, as it is certain that Kippour is holier than Yom tov?
Daniel Ettedgui, Boca Raton, USA
That is an excellent question.
Actually, Eruv Tavshilin is only mid'Rabanan. Had it been prohibited by the Torah to prepare from Yom Tov to Shabbos, no Eruv would permit it. However, since it is only prohibited mid'Rabanan, the Rabanan were able to enact an Eruv to permit preparing from Yom Tov for Shabbos.
As for why the Rabanan prohibited cooking from Yom Tov for Shabbos without an Eruv - the Gemara in Beitzah 15b offers two reasons for the edict: (a) in order to prevent people from preparing on Shabbos or Yom Tov for weekdays, (b) in order to remind a person not to eat all of the food that he prepared for Shabbos on Yom Tov itself, leaving nothing for Shabbos.