Why does census happen every 10 years?

The Census takes place every 10 years. Some states gain more population than others, and others even lost population. Since the 435 seats in the House of Representatives are given on the basis of population (higher population states get more seats), these seats must be reapportioned on the basis of Census results. States that gained enough population must gain more seats, taking them from states that either didn't gain as much population, or lost population.