Explain why you think a country needs rules about who is a citizen and who isn't?

A country needs rules to keep Law and Order without this the countries government system will fail. Proving citizenship is important because of the currency, take Mexican immigrants for example they cross the border illegally and find work on american soil, then the majority send money back to their family in mexico because american dollars are slighty worth more than "pesos" this over time will ruin the american economical system and this is why citizenship is important to a country.