It's April 15th! You might be asking why you're paying income taxes today. The 16th Amendment, ratified in 1913, authorized the federal government to directly tax individual incomes. The 16th Amendment was the first of four "Progressive Amendments," which in addition to authorizing the income tax, resulted in the direct election of senators, prohibition of alcoholic beverages, and women's suffrage.
In general, the Progressive Amendments (16th, 17th, 18th, 19th) strengthened the federal government's authority and went along with Progressive goals of organizing society based on scientific principles, legislating moral improvement, regulating business, and promoting political democracy.