The IRS is responsible for checking if you are paying the correct amount of taxes, and a component of that is knowing what income you have and where it comes fr

The IRS is responsible for checking if you are paying the correct amount of taxes, and a component of that is knowing what income you have and where it comes from - since tax depends on the source of income typically (you have to pay different amounts of tax on money you got by selling shares versus money you stole from a shop).