Taxes are taken from the payslip, but there’s a bunch of other things that can affect your income taxes. For example, you need to pay income tax on bank interest and capital gains tax on investments you sold during the year. There’s also a bunch of deductions that reduce your taxable income - for example, if you have a mortgage, you can deduct some of the interest.

Of course, the IRS should already know most of this stuff, so I’m not sure why they make us tell them again.