The first income tax in the United States was implemented in 1861 to help the federal government cover the costs of the Civil War, according to the Library of Congress. This tax was repealed 10 years later, and after several variations was eventually replaced by the 16th Amendment to the U.S. Constitution, which allowed the government to collect an income tax as a source of federal revenue.
The 16th Amendment was ratified in 1913. Since then, federal income taxes have come to be seen as an indispensable part of what Oliver Wendell Holmes described as the price paid "for a civilized society," according to the U.S. Department of the Treasury. The framers of the Constitution enumerated the government's obligation to provide for the common defense and promote the general welfare. Taxes on income are the method by which the government collects the revenue necessary to execute these functions.
Another useful function of public taxation, according to the Treasury, is in educating the public about the real cost of the government's services. Knowing how much a certain policy will cost before it's implemented encourages lively debate and the rational cost-benefit arguments that are central to policy decisions undertaken by the citizens of a republic. A policy of general taxation thus encourages a broad public understanding of what the government does and how it operates, information vital to voters.