Who pays estimated tax?
Not all income is set up so that taxes are deducted at the source. Independent contractors and freelancers, for example, do not have tax deducted from their pay. Earnings from interest, dividends and rent, taxable unemployment compensation, retirement benefits and the taxable part of your Social Security benefits are other examples of income that is not taxed at the source. If you have any such income coming in, then you should pay the estimated tax.