The Hebrews are called to be set apart because the Lord is holy. As such, they are called to reflect his holiness by distinguishing between the clean and unclean. The emphasis of this command is highlighted by Currid who points out that these verses are structured as a chiasm:
A You shall separate
B the clean beast
C from the unclean
C’ the unclean bird
B’ from the clean1
25 You shall therefore separate the clean beast from the unclean, and the unclean bird from the clean. You shall not make yourselves detestable by beast or by bird or by anything with which the ground crawls, which I have set apart for you to hold unclean.