Two major reasons for the U.S. Census is to collect population data that is used for redistricting and apportionment. The Enumeration Clause of the Constitution requires that apportionment be based on a decennial population count. However, the Constitution does not state where redistricting data must come from. It has become standard practice for states to use Census data for this.
Apportionment is the process of dividing the 435 seats in the U.S. House of Representatives among the states. The Census uses an "Equal Proportions Method" equation to get the results. Here is a map of 2020 Census apportionment numbers for each state.
Legislative redistricting is currently underway in the United States. In the process of redistricting, the Census Bureau provides population data to all the states, District of Columbia and Puerto Rico so they can redraw congressional and state legislative boundaries. Public Law (P.L.) 94-171, enacted by Congress in December 1975, requires the Census Bureau to give states the chance to "identify the small area geography for which they need data in order to conduct legislative redistricting."
Under the normal timeline, apportionment data was scheduled to be delivered to Congress on December 31, 2020. However, because the pandemic caused delays, the data was not delivered until April 26, 2021. Also, redistricting data was scheduled to be released on April 1, 2021, but wasn't released until August. This has given states less time to redistrict in time for elections next year and some states have constitutional or statutory deadlines for redistricting. The pandemic has caused delays in the whole process.
Redistricting and the Use of Census Data
Searching for Silver Linings in Delayed Census Results