Being able to reference the input from a fillable field as the value for a Variable via the "Field Id" would allow for more complex functions like being able to do basic math within a document utilizing input from multiple fields. You could also use fillable field input to flow into a pricing table, which would make this even easier to implement.

My personal use case is needing to repurpose existing PDF contracts currently in use, but having the limitation of not being able to insert a pricing table in order to provide totaling, tax/discount calculations, etc from multiple fillable fields. This way, they could be referenced into a pricing table on a following page, tabulated, and then the output sent via Pricing Table Variable to a text box.