From the Document: "The census is a count, as nearly complete and accurate as possible, of every person whose usual residence is in the United States. Article I, Section 2, clause 3 of the U.S. Constitution, as modified by Section 2 of the 14th Amendment, requires a population census every 10 years, conducted 'in such Manner as they [Congress] shall by Law direct.' Congress, in Title 13, 'U.S. Code', has delegated this responsibility to the Secretary of Commerce and, within the U.S. Department of Commerce (DOC), the Census Bureau. The first census took place in 1790; the next will be in 2020. The constitutional reason for taking a census is to have an updated basis for apportioning seats in the U.S. House of Representatives. Census data also are used to redraw legislative boundaries within states; to produce population estimates and projections; in formulas that help allocate more than $675 billion in federal funds annually to states and localities; and by subnational governments, businesses, nonprofit organizations, and researchers for myriad purposes."
CRS In Focus, IF11015
Congressional Research Service: https://crsreports.congress.gov/