Why do we say Harei At Mekudeshes Li … “k'Das Moshe v'Yisrael”?

Question

Why do we have this "k'Das Moshe v'Yisrael" addendum?

To my knowledge we don't really say this by other mitzvos so why do we say it here/ what's it adding?

Answer 1

Tosafos in several places explain that we say k'das Moshe v'yisrael by marriage because marriage is effective contingent upon the approval of the Sages. In certain instances where the Sages disapprove of certain actions, they render the marriage as if it had never occurred.

Gittin 33a (My emphasis)

ומי איכא מידי דמדאורייתא בטל גיטא ומשום מה כח ב"ד יפה שרינן אשת איש לעלמא אין כל דמקדש אדעתא דרבנן מקדש ואפקעינהו רבנן לקידושין מיניה אמר ליה רבינא לרב אשי תינח דקדיש בכספא קדיש בביאה מאי איכא למימר שויוה רבנן לבעילתו בעילת זנות

And is it possible then, that where a Get is according to the Written Law cancelled we should, to save the authority of the Beth din, [declare it valid and] so allow a married woman to marry another? — Yes. When a man betroths a woman, he does so under the conditions laid down by the Rabbis, and in this case the Rabbis annul his betrothal. Said Rabina to R. Ashi: This is quite right if the husband had originally betrothed his wife with money. But if he had betrothed her by the act of marriage, what can we say? — The Rabbis declared the act of marriage to be retrospectively nonmarital. (Soncino translation)

Tosafos s.v. Kol (commenting on the emphasized words above)

ולכך אומר בשעת קידושין כדת משה וישראל

And therefore we say at the time of the marriage "in accordance with the law of Moses and Israel".

Note that in the text of a Get the same phrase appears, as recorded by Rambam in Hilchos Gerushin 4:12:

ודן די יהוי ליכי מנאי ספר תרוכין וגט פטורין ואגרת שבוקין כדת משה וישראל

This will serve you as a bill of divorce, a get dismissing you, and a letter releasing you, from me, according to the faith of Moses and Israel. (Touger translation)