Each of the states has its own courts, it's own governments, and its own powers to police within its state borders. The Constitution provided certain "enumerated" powers to the federal government, including the following: The power to impose and collect taxes which are to provide for the debts of the United States, as well as for the common defense and welfare of the country. All such taxes are to be implemented equally throughout the nation.
The regulation of commerce, both on the international and interstate levels.
The power to establish currency and coin money.
The power to establish post offices.
To provide and maintain a navy.
Organize, train, and arm a militia.

Please explain why it might be in the interests of the country for these powers to be given to the federal government rather than the states. For example, what would happen if each state established its own army/militia? What would be the result if each state had its own currency and a New York dollar was not accepted in other states like Delaware?