Ramban: After issuing a warning about the Mitzvos in general - represented by the La'av of Avodah-Zarah, ¹ Moshe warns the people about the Chukim - the statutes, and Mishpatim - the civil laws, which require special attention - the former, because the reasoning behind them is unknown; the latter, because 'the land continues to exist (solely) upon justice'. Moreover, Moshe is pointing out to them the special qualities of the Chukim and Mishpatim, in that a. they elevate those who observe them and that even their enemies praise them (Yisrael) on account of them, ² b. they posses the incredible benefit that, because of them, Hashem answers them whenever they call out to Him and c. they are intrinsically righteous and good, for the survival of the people and the community at large. ³

Yerushalmi Nedarim, 4:3: With reference to 4:5:2:1 - In order to confine the obligation of teaching free of chartge to Chukim and Mishpatim (Torah she'be'al Peh) but not to Mikra and Targum (Torah she'bi'Kesav') for which one may take payment. ⁴

Bechoros, 29a: It implies ¹ that, just as Moshe learnt from Hashem free of charge, so too should others learn from him free of charge ² - prohibitting a Rebbe to take S'char Limud from his Talmidim.

Oznayim la'Torah: To counter the argument of Korach, who claimed that Moshe spoke off his own bat and that Hashem did not send him.

Ramban: Either because many of the Mitzvos only apply in Eretz Yisrael, or because basically, all the Mitzvos are meant to be kept specifically in Eretz Yisrael. ¹

Oznayim la'Torah: To counter the argument that teaching free of charge is feasable in the desert, where the Manna fell from heaven and the well provided them with water, but not in Eretz Yisrael, where one needs to work for a living.