The Census is required by the U.S. Constitution for determining how many representatives each state will have.
Article 1, Section 2
“The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct.”
14th Amendment, Section 2
“Representatives shall be apportioned among the several States according to their respective numbers, counting the whole number of persons in each State, excluding Indians not taxed.”