A PDF form with clickable fields is convenient to fill, but those fields stay editable — anyone who opens the file afterwards can change your answers, and some viewers render fields inconsistently. Filling and then flattening gives you a clean, final document where the values are printed onto the page for good.
Two steps, two tools
First fill the form: the fields are detected right in your browser, so you type your answers directly into the document. Then run the result through the Flatten tool, which turns the filled fields into permanent page content — no longer clickable, no longer changeable.