Reason #2:  POLITICAL POWER  Aside from gathering information of immense practical and historical value, the Decennial Census is used to determine how many seats each state will be given in the U.S. House of Representatives.  States also use the information to realign congressional and state legislative districts in their states, taking into account population shifts since the last census. 

 “The Founders of our fledgling nation had a bold and ambitious plan to empower the people over their new government. The plan was to count every person living in the newly created United States of America, and to use that count to determine representation in the Congress.

Enshrining this invention in our Constitution marked a turning point in world history.”    

                                                                                                                                  –U.S. Census Bureau

Reason #3:  MONEY   Over 400 billion of dollars in federal funds are allocated to states and local communities based on population.

That's an unimaginable amount of money! States, cities, oounties and townships can make use of that money, so the arguments over how many people live in a given area can sometimes get fierce. By the way...that number is soooo big that if all you did was to just count aloud to 400 billion, it would take you about 96,000 years!!