The Mitzva of Writing a Torah Scroll
מצוות כתיבת ספר תורה
It is a positive Torah commandment for every Jew to write a Torah scroll for himself, as it says: “Now therefore write this song for yourselves” (Devarim 31:19). The Torah’s intent is to command each and every Jew to write for himself the entire Torah, which includes the song of Ha’azinu. Even a person who inherited a kosher Torah scroll from his forefathers does not fulfill his obligation with it; rather, he is commanded to write the Torah himself (Sanhedrin 21b). Whoever fulfills this mitzva is regarded by Scripture as if he received the Torah at Mount Sinai (Menaḥot 30a).
The choicest way to fulfill the mitzva (mitzva min ha-muvḥar) is to write the Torah himself, in ink on parchment. But if he does not know how to write, or he is busy with his work and has no time to write the Torah, he can fulfill the mitzva by hiring a scribe to write the Torah for him, and in this way too he fulfills the mitzva in the choicest manner. However, if he buys a Torah scroll whose writing has already been completed, although he has fulfilled the mitzva, he has not fulfilled it in the choicest manner, since the scroll was not written as his agent; and the Sages said that he is like one who snatches a mitzva from the marketplace. Therefore, even one who does not want to trouble himself with finding a scribe, but rather wants to buy a ready-made scroll, should buy a scroll that is still missing a few letters, and the scribe should write them as his agent; in this way he will be a partner in completing the writing of the Torah (Menaḥot 30a; Rashi and Nimukei Yosef; Bi’ur Ha-Gra 270:3; Arukh Ha-shulḥan 270:2; and see Rema, YD 270:1).
Two main reasons have been given for this mitzva. The first is that writing the Torah in ink on parchment, according to the tradition handed down from one person to the next all the way back to Moshe at Sinai, constitutes a continuation of the Torah. As the Sages said, whoever fulfills this mitzva is regarded by Scripture as if he received the Torah at Mount Sinai (Menaḥot 30a).
The second reason is so that Torah scrolls will be available to every Jew, and they will be able to study from them constantly, so that they merit to know God and fulfill His mitzvot. Even if a person inherited a scroll from his parents, it is possible that studying from the old scroll will be difficult for him; therefore, the Torah commanded that each person write for himself a scroll from which he will study all the days of his life. So wrote Sefer Ha-ḥinukh (mitzva 613).