On Shabbat morning, the fourth Amidah blessing includes Yismach Moshe:
"Moshe rejoiced in the gift of his portion: That You called him a faithful servant. A crown of splendor You placed on his head when he stood before you on Mount Sinai. He brought down two stone tablets in his hand, on which is inscribed the observance of the Sabbath. So it is written in Your Torah..." [then the Veshameru]. [ArtScroll translation]
(1) It is odd. Why is it there? What would be lost if it were removed and we went right to Veshemeru, which is a Torah quote [Ex. 31:16-17] that is more illuminating?
(2) Who composed it and when was it introduced in the siddur?
Background note: Rashi wanted it out, but he was obviously ignored. In Sefer Manhig, Hilkhot Shabbat #20, p. 150, we see: Rashi would not say Yismach Moshe, but would say “You chose us.” He would say: “Hashem, our God, gave us Sabbaths for rest.” Because he didn’t know what the connection was between Shabbat and Yismach Moshe!.. But [his grandson] Rabbeinu Tam restored the matter to its original glory.