But of course the IRS probably isn't the only entity to which you owe taxes (states and the federal government are separate entities), so the IRS's forms only g

But of course the IRS probably isn't the only entity to which you owe taxes (states and the federal government are separate entities), so the IRS's forms only get you part of the way. What services like Credit Karma do is let you input your W-2 forms or similar (which indicate taxes paid), ask you questions to determine any adjustments you might need/want to make, generate the correct forms to send to the IRS and your state tax agency, and transfer the money you owe from your bank account. Doing all that by hand is quite a bit more work.