According to the United States Constitution, the federal census is used to determine the 3. apportionment of members in the House of Representatives. Unlike the Senate, which allocates an equal number of two Senators to each state, the House of Representatives is made up of members who represent districts. These districts are ideally made up of equal populations, a number that can change decade to decade. Since the census shows how many people live where, it is used to determine the population of representative districts. If a lot of people move to an area, odds are they will gain a seat in the House; however, that means a lot of people left a different area, which means the area they left would likely lose a seat in the House.
