Because we have a group of united states. States have their own laws and government, but have ceded some powers to the Federal government (foreign affairs, defense, etc.).
Also, for the same reason we have verbs. We need them, and they make things clearer.
Think about it for a moment. Is it easier to take on a large undertaking as a whole, or by breaking it into parts. By sectioning the country into states, it makes governing the country easier. The Federal Govt presides over the 50 broken up chunks, the states themselves have Governors to preside over the state and it’s counties as a whole, inside your counties you have cities that are governed by mayors and then households governed by head of households and then individuality. It makes keeping a massive country such as ours in check a bit easier.