Congress creates, makes, distributes, and maintains the currency for America. When America was a new country every state maintained its own currency. This caused many problems including the fact that money become useless because anyone could copy it and there was nothing of value to back it up. To fix this, congress decided to use gold to give the countries's money value.
Congress was charged with creating a Navy. America was an isolated country surrounded by Natives. The biggest threat to the country at the time was invasion from European countries. In order to protect the borders, Congress created and maintained the Navy.
Trade between states and countries needed to be regulated. Although the states regulate trade between them, it is the responsibility of Congress to regulate trade between America and other countries.