Why does the Torah need to mention that Reuven was the firstborn of Yisrael?
Seforno: To teach us that, in spite of his sin, due to the fact that he did Teshuvah, be'Dinei Shamayim, Reuven retained his status of B'chor.
Da'as Zekenim and Rosh: Because, since Yehudah traveled first, the Torah needs to explain why Reuven is listed first.
Oznayim la'Torah: To teach us that, although Reuven lost the monetary rights of a B'chor, his Yichus as the B'chor remained intact.
Why does the Torah write "le'Gulgelosam" only in connection with Reuven and Shimon, but not in connection with the other tribes?
Da'as Zekeinim and Rosh: Because Ya'akov rebuked them 1 when he blessed his other sons, and they needed a Kaparah - and in connection with the half-Shekalim, which came as a Kaparah, the Torah wrote "la'Gulgoles" (Sh'mos 38:26).
Riva: Because they needed a Kaparah ? Reuven for moving his father's bed, and Shimon, either for marrying Dinah, or - according to the opinion that a Ben Noach may marry his full sister, for the episode with Yosef and for killing Sh'chem. 2
Oznayim la'Torah (citing the G'ra): For the sake of brevity, the Torah mentioned it by the first two tries and relied on that regarding the other tribes.
