It is a Constitutional requirement that every person in the United States be counted at least once every 10 years. The census started in 1790. The main purpose of the census is to count the national population to determine representatives in the United States Congress. The census data also provides funding guidance to federal assistance programs.
Looking for older census documents? Search HathiTrust at: https://www.hathitrust.org/. HathiTrust contains thousands of digitized government documents.