Imagine a form that is split into two sections, each of which has to be filled out and signed by a differemt person. The first person filling and signing will implicitly also sign the empty fields of the second person, and the second person will then change those fields to have actual values. The author of the form can specify which fields are allowed to be changed after which signature, and a validator would have to check that those constraints were observed.

EDIT: You may have come across forms with fields marked with something like “don’t fill out this field, it will be filled in by the clerk processing the form”. That’s one kind of use case.