1913 the sixteenth amendment which legalized a federal income tax was ratified why was there a need for a federal income tax

The need for the federal income tax actually dates back further than 1913, to the period just after the start of Civil War. At that time, the government levied a tax in order to help pay war expenses, and that tax became part of the government's operational foundation going forward. A number of various tax schemes were put in place and eventually declared unconstitutional, motivating the need for an amendment which was eventually put forth in 1913.