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(19) Now therefore write this song for you, and teach it to the children of Israel; put it in their mouths, that this song may be a witness for Me with the children of Israel.
(א) לכתב כל אחד ספר תורה לעצמו - שנצטוינו להיות לכל איש מישראל ספר תורה...
משרשי המצוה, לפי שידוע בבני אדם שהם עושין כל דבריהם לפי ההכנה הנמצאת להם, ועל כן צונו ברוך הוא להיות לכל אחד ואחד מבני ישראל ספר תורה מוכן אצלו שיוכל לקרות בו תמיד ולא יצטרך ללכת אחריו לבית חברו.
For each person to write a Torah scroll for oneself - we have been commanded for each person in Israel to own a Torah scroll...
Among the roots of this mitzvah, it is because it is well-known among people that they do everything according to that which they find prepared for themselves, and therefore God commanded us, each and every one of the Children of Israel, to have a Torah scroll prepared and available to him, so that he can read it always and will not have to go looking for it at his friend's house.
אמר רבא אף על פי שהניחו לו אבותיו לאדם ספר תורה מצוה לכתוב משלו שנאמר (דברים לא, יט) ועתה כתבו לכם את השירה
Rava said: Even if one's parents left him a Sefer Torah, it is a mitzvah to write one of his own, as it is written: Now therefore write this song for you.
ספר ערוך השולחן :הקדמה על חושן משפט
וכל מחלוקת התנאים והאמוראים והגאונים והפוסקים באמת למבין דבר לאשורו – דברי אלקים חיים המה, ולכולם יש פנים בהלכה. ואדרבה: זאת היא תפארת תורתינו הקדושה והטהורה. וכל התורה כולה נקראת "שירה", ותפארת השיר היא כשהקולות משונים זה מזה, וזהו עיקר הנעימות
Aruch Hashulchan: Introduction to Chosen Mishpat
And all the disputes of the Tanaim, and the Amoraim, and the Gaonim, and the poskim that try to understand a topic properly are the words of the living God. And they are all different sides of the Halacha. This is the glory of our holy and pure Torah. And all of Torah is called a "song" and the splendor of song is that everyone's voice is different. This is the essence of its beauty.
וכתב אדוני אבי הרא"ש ז"ל שזה לא נאמר אלא לדורות הראשונים שהיו כותבין ספר תורה ולומדים בה אבל האידנא שכותבין ספר תורה ומניחים אותו בבית הכנסת לקרות בהם ברבים מצות עשה על כל ישראל אשר ידו משגת לכתוב חמשה חומשי התורה ומשנה וגמרא ופירושיהם להגות בהן הוא ובניו כי מצות כתיבת התורה היא כדי ללמוד בה דכתיב (דברים לא: יט) ולמדה את בני ישראל שימה בפיהם
Arbaah Turim 2:270
My master, my father, the Ro"sh, of blessed memory, wrote that this was written only for the earlier generations, who would write a Torah scroll and learn from it. But nowadays, when we write a Torah scroll and place it in the synagogue to read from it publicly, it is a positive mitzvah incumbent on all Jews who have the means to do so to write volumes of the Torah, and Mishnah, and Gemara, and their commentaries, and to read from them - him, and his sons. For the mitzvah of writing a Torah is in order to learn from it, as it says: (Deut. 31:19) "and teach it to the people of Israel, place it in their mouths."
Rabbi Jonathan Sacks
We have to write our own scroll. The point about the Torah is not that it is old but that it is new; it is not just about the past but about the future. It is not simply some ancient document that comes from an earlier era in the evolution of society. It speaks to us, here, now - but not without our making the effort to write it again. There are two Hebrew words for an inheritance: nachalah and yerushah/ morashah. They convey different ideas. Nachalah is related to the word nachal, meaning a river, a stream. As water flows downhill, so an inheritance flows down the generations. It happens
naturally. It needs no effort on our part. A yerushah / morashah is different. Here the verb is active. It means to take possession of something by a positive deed or effort. The Israelites received the land as a result of God’s promise to Abraham. It was their legacy, but they nonetheless had to fight battles and win wars. Lehavdil, Mozart and Beethoven were both born to musical fathers. Music was in their genes, but their art was the result of almost endless hard work. Torah is a morashah, not a nachalah. We need to write it for ourselves, not merely inherit it from our ancestors....