The shirah, song, to which the Torah is referring is Shiras Haazinu, which is called a song because it is written in the form of poetic verse. The Rambam (Hilchos Sefer Torah 7:1) implies from here that it is a mitzvah for every Jew to write a Sefer Torah for himself. Although the Torah here says only to write a song, it is incumbent on us to write the entire Torah. It seems that the Torah is being written in order to write Shiras Haazinu. Why should the mitzvah of kesivas Sefer Torah, writing a Torah scroll, be presented in such a roundabout manner?
Furthermore, we are allowed to write the individual parshios of Tefillin and Mezuzah which are later inserted in their cases. What about the prohibition of writing individual parshios? Furthermore, in the time of the Bais HaMikdash, the Parashas Sotah, dealing with the wayward wife, is written separately. What is different about Shiras Haazinu that it cannot be written unless it is part of an entire Sefer Torah?
Horav Moshe Feinstein, zl, suggests that we focus on the reason stated by the Torah for writing Shiras Haazinu, “So that this song shall be for Me a witness against Bnei Yisrael.” The function of the song is to bear witness, to attest to the Jewish People’s observance of the Torah. Clearly, in order to bear witness, it is incumbent that we be proficient in the entire Torah, concerning which the Shirah will testify. Without the Torah, we would be hard-pressed to know that when Hashem decrees punishment, it is in retribution for our lack of compliance to the Torah’s demands. Hashem’s will is stated in the Torah. He informs us what He expects of us. If we have no Torah, we can hardly know the correct manner of observance. Without the “book of directions,” it is difficult to use the appliance appropriately.
Without Torah as one’s guide, his desire to grow and excel spiritually will be impeded. On the contrary, he may stray (through no fault of his own, other than a lack of Torah knowledge) and perform totally wicked actions. The generation of Enosh, who began worshipping idols, made the grave error of thinking that it was Hashem’s will that they worship His celestial bodies. Perhaps they meant well, but, without the illumination of the Torah, they groped in the darkness until they perpetrated the most infamous treason against Hashem. This misconduct is the product of Torah ignorance. Such unawareness, unfortunately, produces similar results in any other area that we might attempt to serve Hashem.
Thus, explains Rav Moshe, the command to write the song is really a command to write the entire Torah, to study it and become fully proficient in it. In this manner, when we come across the punishments expressed in the song which are the consequences of our lack of observance, we will be able to look in the Torah and learn how to expiate our sins and correct our ways.