Income Tax
"The congress shall have power to lay and collect taxes on incomes, from whatever source derived, without apportionment among the several states, and without regard to any census or enumeration."
In essence: The United States government required ways to raise funds; the civil war had exposed financial weakness of the united states government. To counter that development, the power to tax income was given to congress. It is still required that the House of Representatives and not the Senate pass such laws, however.