There are many expressed or enumerated powers that the United States Constitution establishes for Congress.
Article One of the Constitution addresses the legislative branch of government. The framers had specific designs for what each branch of government could do. They wanted to ensure clear communication and the ability to use governmental power properly.
In section eight, the first enumerated power regards taxation. Congress has the
"power to lay and collect taxes, duties, imposts and excises, to pay the debts and provide for the common defense and general welfare of the United States; but all duties, imposts and excises shall be uniform throughout the United States."
It makes sense that this is the first stated Congressional power. Unfair taxation was one of the reasons why the colonists fought Britain in the American Revolution. The framers, many of whom fought as colonial patriots against the British, did not want to experience the same abuse. By having Congress control taxation, they figured power would be less likely to be abused if it was shared amongst more than 500 people, rather than placed in the hands of one president.
There are many other powers that the Constitution explicitly grants to Congress. However, I think that one of the most important would be "To declare war, grant letters of marque and reprisal, and make rules concerning captures on land and water." The president is the Commander in Chief, but the nation can only enter war with Congressional approval. The Framers were quite insightful in understanding that one of the best ways to limit the machinery of war was to mandate its approval through the legislature. Once again, the desire to disperse power amongst more people becomes evident in the legislative branch's expressed power to declare war.