1)
What is this (otherwise superfluous) Pasuk coming to teach us?
1.
Seforno: It teaches us that the B'nos Tz'lofchad married men of their tribe, not because they desired them, but because that is what Hashem commanded Moshe.
2)
Why does the Torah switch the order? Why does it not write va'Ya'asu B'nos Tz'lofchad ka'asher Tzivah Hashem ... '?
1.
Oznayim la'Torah and Or ha'Chayim: To teah us that the B'nos Tz'lofchad went out of their way to negate their own natural instincts - to marry whom they desired - before the Will of Hashem.