That's funny, my friend just fills out his taxes as soon as they are eligible to be submitted with rough guesses. The IRS sends the forms back to him corrected (they already had the correct form) and then he mails them back again.

The US already has pre-filled forms, the interface for the "pre fill" button is just a bit convoluted.

So really they already know what they expect you to write in your tax forms, they just want you to do this as if it was a written exam?

Yes, it's basically a written exam. The grading is done as a free service by the government, or a handout if you will.

They give you a chance to declare income that they may not know about at that very moment.

It is a strong incentive to not lie on your taxes