The basic purpose of the Census is to count the population within each state, so that congressional seats can be proportional. In a representative democracy, states with large populations should have more members in the U.S. House of Representatives than smaller states.
Over time, however, the use of the Census has expanded. It has been used as a tool to make sure that congressional and other voting districts include a representative population under laws, such as the Civil Rights Act of 1964 and the Voting Rights Act of 1965, enacted to ensure that the voting rights of Black Americans were protected.
The decennial Census also serves as the population base for subsequent American Community Survey releases, which are used to distribute trillions of tax dollars over a decade. Because it’s an actual headcount—as opposed to a survey—the decennial Census creates a foundation for the annual ACS data.