Oznayim la'Torah: Because Eisav might have argued later that the sale was invalid since he was forced to agree to the sale due to life-threatening hunger. Therefore, Ya?akov gave to him bread, and after ¹ he ate and was satiated, he sold his Bechorah for the stew, demonstrating that he sold it due to contempt of the Bechorah.

Alshich: He was teaching him that it is gluttonous to pour the cooked food down his throat without bread, like one feeds camels. Rather, he should eat slowly with bread, like humans.

Maharal (Netzach Yisrael Ch. 15, p. 89): Esav wanted to sell the Bechorah due to his physical nature (i.e. he had no desire for Kedushah). He therefore received payment in material terms.

Rashi: It means that Eisav disparaged the Avodah of Hakadosh-Baruch-Hu ¹ - to teach us how wicked he was.

Ramban: The Pasuk is informing us why Eisav was burned so badly on account of the Bechorah, as the Pasuk writes in Mishlei, 13:13 "He who scorns something will ultimately be injured by it".

Seforno: Even after selling the Bechorah, he continued to consider it of no importance and not worth the price that Ya?akov paid for it - which teaches us in turn, that Ya?akov was not guilty of cheating Eisav.

Rashbam: The Torah tells us now how he despised the Bechorah, as a prelude to the story later, where he changed his mind and complained bitterly to his father "He stole my birthright!"

Da'as Zekenim: It teaches us that he despised the Bechorah even before the current episode, ²

Ohr ha'Chayim and Malbim: It teaches us that the sale was not amidst duress of dire hunger. Also after he ate, he disparaged the Bechorah.

Ha'amek Davar: Eisav openly held that the birthrite does not depend on order of birth, rather, on attributes. Many non-Jewish leaders held like this.

Refer to to 25:34:2:1-7.

Ibn Ezra: Because at that time, Yitzchak was poor, and he saw no financial advantage in being a firstborn. ¹

Maharal (Ohr Chadash, p. 130): Esav himself is "disgraced" (Ovadyah 1:2), and as such he disgraces others.