During the repetition of the amidah, during the hoda'ah (modim) the congregation recites a different text while the shaliach tzibbur recites the text we all did during the silent amidah. (1) Why do we recite a different (related) text instead of listening to the sha"tz, and (2) why only here?
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Abudraham, in "Seder Shacharit shel Chol u-Ferushah", says( here, right column, lines 29-38):
In my translation:
I heard in a recording of R' Yisroel Simcha Shorr that this is learned from the parsha of Bikkurim. Bikkurim is a parsha of hakaras hatov - we are thanking Hashem for the good of the land that He gave us - and the halacha is (Mishna Bikkurim 1:5) that the parsha of Bikkurim cannot be read by proxy. From this we learn that you cannot give thanks appropriately through a messenger.
(R' Shorr didn't give a source for this idea.)