Gitin, 60a: It implies that, even if Moshe wrote each Parshah as soon as it was taught to him, ¹ he now sowed the Parshiyos together into one Seifer. ²

Rashi: The Chachamim argue in Bava Basra, 14b, as to whether they fixed a board on to the outside of the Aron (R. Yehudah), on which they placed it, ¹ or whether they placed it inside the Aron (at the side - R. Meir).

Ramban: He placed it inside the Aron at the side. ²

Da'as Zekenim, Rosh (citing the Ramban, both citing a Midrash), Targum Onkelos and Targum Yonasan: He placed it next to the Aron ³ (on the right hand side in a box - Targum Yonasan).

Seforno: It testified (throughout the ages) ¹ that Hashem knew in advance that they would forsake His Torah, ² and that it would corroborate everything that is written in in Sifrei Torah of Tzadikim of the generation - a proof that, word for word, that is what Hashem told Moshe at Sinai.

Oznayim la'Torah: It would testify that this is the authentic text - to counter any attempts to make any changes to it - to fit in with the times. Hence it was placed in the safest of locations where it could not be tampered with.

Rashbam: Refer to 31:21:1:1*.

Seforno: So that Yisrael should attribute the ensuing punishments to their sins and not to chance.