After defeating the enemy, why should anyone proceed to worship their gods?
Ramban: Due to the severity of the Isur, 1 the Torah tends to warn against it at every opportunity, even when it appears superfluous. It inserts it here, to warn against it even when, after entering Eretz Yisrael, we would find the Cana'ani idols already made and ready to be worshipped.
Seforno: In order to appease them for having killed their adherents. 2
What are the connotations of "ve'Lo Sa'aseh ke'Ma'asehem"?
Ramban #2: More likely, after warning against worshiping it in a way that is Derech Kavod (such as bowing down to it), it is a prohibition against worshipping it in the conventional manner, but which is not done Derech Kavod. 3