The Revenue Act of 1861 introduced a federal income tax. The first, actually. But eventually the tax was repealed in 1872. During the late 19th century, various groups, the Populist Party being one of them, favored the establishment of a progressive income tax at the federal level. Those groups had the goal of having the tariffs being more fair instead of unfairly taxing the poor. They favored using the income tax to shift the tax burden onto wealthier individuals.
It established the federal government's right to impose a graduated income tax.
It helped create economic fairness.