What does the Torah mean when it writes "Ki Li Chol Bechor ... "?
Rashi: It means 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 Leviyim, who did not sin.
Seforno: It means that, initially, the Bechoros were automatically destined to perform the Avodah, since they were the most honored members of their respective families.
Why did Hashem find it necessary to acquire the firstborn to save them from Makas Bechoros?
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 they are redeemed. 1
Seforno: As we saw in , 3:45.