Workpapers are necessary to support the entries on the books and in the tax return

The Internal Revenue Code requires every taxpayer to keep records and
statements necessary for the proper administration of the tax laws.
The accounting records of a taxpayer must include the regular books of account, as well as any working papers that may be necessary to support the entries on the books of account and in the tax return.

However, nothing in federal tax law prescribes a specific format for tax return working papers.

Updated for the Tax Cuts and Jobs Act (TCJA), this webcast will help you understand, prepare and organize workpapers.