ועתה כתבו לכם את השירה הזאת ולמדה את בני ישראל
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…