The employer tax withholding does not account for any other source of income you may have (sell stocks, rental income, make money on Twitch/Youtube/ebay/etc, wi

The employer tax withholding does not account for any other source of income you may have (sell stocks, rental income, make money on Twitch/Youtube/ebay/etc, withdrawal from tax free investment, interest, etc) so you may need to pay more taxes than they withheld and at the same time it doesn't account for all the possible deductions you may have that year so it may withhold more than you need to pay (usually that's the "desired" case). So at the end of each year you need to go through all the income sources of that year, subtract all deductions and compute how much actual tax you own. And pay or receive the difference to/from IRS. Even if you have an extremely simple income situation (only one wage, no other income), depending on the state you live in you may still qualify for deductions that the employer is not aware of so it can be in your advantage to do the taxes.