How does blur background differ from portrait mode?
A phone's portrait mode measures depth while the picture is taken, so objects far away get softer than those just behind you. This tool works on a finished photo and has no depth information: it separates the subject from everything else and blurs the rest evenly.
For most portraits and product shots the result reads the same. Where it shows is in a scene with a floor or table running toward the camera, which a real lens blurs gradually and this tool blurs all at once. If that looks flat, a lower Blur strength usually hides it.
