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?

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    @GershonGold Thanks! Saw this- it answers that k'daas Moshe v'Yisrael wouldn't be me'akeiv but doesn't really answer my question- namely, that why specifically by kiddushin we have this lingo, but not by other mitzvos? – alicht Jan 4 '19 at 2:27

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)

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    Thank you so much! Such a sound svara and thanks for refreshing my memory about "ואפקעינהו רבנן לקידושין מיניה" (I think there's also a Tosfos in the beginning of kesuvos about ואפקעינהו) – alicht Jan 4 '19 at 3:28
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    @alicht You're welcome. The same Talmudic passage appears in several places, one of them being Kesuvos 3a. Tosafos there makes the same comment. – Alex Jan 4 '19 at 3:30
  • follow up question- why exactly is כל דמקדש אדעתא דרבנן מקדש? Why does this principle exist? Is it d"oraisa or d'rabanan? – alicht Jan 4 '19 at 3:35
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    @alicht Check out the post I linked in my answer. If that doesn’t help you, you should post a new question detailing what specifically you’re looking for that’s not covered by that question. – Alex Jan 4 '19 at 3:48
  • Thanks! Just asked it in a separate question judaism.stackexchange.com/questions/98220/… – alicht Jan 4 '19 at 4:15

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