Rashi: It implies that when Hashem smote all the firstborn in Egypt, He took the Jewish firstborn under His Wing to protect them - thereby acquiring them, until they sinned by the Eigel, when He switched them for the Levi'im, who did not sin.

Seforno: It implies that initially, the Bechoros were automatically destined to perform the Avodah, since they were the most honored members of their respective families.

Bechoros. 4b: It teaches us that they sanctified the firstborn in the desert. ¹

Yerushalmi Megilah, 1:11: In conjunction with the Pasuk in Sh'mos, 12:12 "Vehikeisi Kol B'chor ... u've'Chol Elohei Mitzrayim E'eseh Shefatim," ² it implies that before the Mishkan was erected, the Avodah was performed by the Bechorim.

Oznayim la'Torah (in Pasuk 16): Hashem acquired the Bechorim for two reasons; 1. because a B'chor is instrinsically special - which explains why Ya'akov made a point of acquiring it from Eisav and why Ya'akov said to Reuven "Re'uven B'chori Atah ... : (before Hashem sanctified the Bechorim in Egypt); ¹ 2. before Makas Bechoros in Egypt, when He acquired them in order to save them from Makas Bechoros. ²

Seforno: Because they were not worthy of being saved from the 'band of evil angels' that attacked Egypt during Makas Bechoros; so Hashem sanctified them, thereby prohibiting them from performing any mundane work - in the same way as He forbade shearing and working with a firstborn animal - until it has been redeemed. ¹

Oznayim la'Torah: Because since once the destructive angel receives permission to destroy, he does not differentiate between Tzadik and Rasha, Hashem sanctified the firstborn of Yisrael - and the destructive angel has no jurisdiction over Hekdesh.