Comment by dpifke

From https://www.irs.gov/e-file-providers/free-file-fillable-form...:

Attaching statements — This program does not allow you to attach any documents to your return, except those available through the program. If you need to attach any such documents, you will have to print and mail in the return.

Especially with business returns, it's common to have to attach statements to certain forms, for example if you choose the de minimis election for equipment repairs that would otherwise have to be capitalized, or when the § 174 changes went into effect and everyone had to start amortizing software development expenses over multiple years. You can't do that on Free File Fillable Forms.