Taxes can get extremely complicated depending on your situation. In my relatively simple case as a contractor, I’m on the hook for calculating and paying taxes quarterly (since my employer(s) don’t do it for me). The exact amount I’m meant to pay is yearly income/4, which means I need to predict how much I’ll earn throughout the entire year, which is impossible without a crystal ball.

So I just pay what I think I’ll earn, and at the end of the year calculate the difference from what I actually earned, and either send or receive a payment from the government. From the IRS point of view, whatever interest they end up paying me is probably less than the cost of government having a cashflow issue throughout the year without those quarterly payments.