A graduated ND filter is useful when you need to control light before it hits the sensor. Software can darken part of an image afterward, but it cannot restore highlight detail that was already clipped.

Key differences:

• physical grad ND: lets you see the effect while composing and can prevent overexposure in bright parts of the scene.
• software grad effect: flexible in strength, size, shape, and angle, but only works well if the bright area still contains recoverable detail.
• exposure blending/HDR: can simulate a grad very well and is highly adjustable, but movement between frames can cause artifacts.

So a real graduated ND filter is most helpful for high-contrast scenes, especially when you want a single exposure and need to protect highlights at capture time. Software is often better when the scene is static and you want maximum flexibility afterward.

More generally, any real ND filter also reduces total light, which software cannot do. That matters when you need a longer shutter speed or wider aperture in bright conditions.