Make a fillable PDF read-only by baking in its field values
A fillable PDF keeps its field values in a separate interactive layer, which any reader can change. Flattening draws those values permanently onto the page and removes the interactive layer, so the document reads the same everywhere and can no longer be edited through the form.
Before sending a completed form to someone else. It guarantees they see exactly the values you entered, stops accidental edits, and avoids readers that render form fields inconsistently or fail to print them.
Not on the flattened file — the interactive fields are gone for good. Your original upload is never modified, though, so keep it if you might need to change the answers later.
The tool tells you so rather than pretending it did something. A PDF with no interactive form is already effectively flat.