depends on how it is used, but in the majority of cases where an ND Filter is actually used, it is because the post-processing equivalent is not sufficient. Example here would be long-exposure (slow shutter) shots during the day. No amount of work in photoshop would salvage a photo taken during the day using a 10-second exposure. If you are wondering if you ever need a 10-second exposure during the day if there is already plenty of light – then that would be where you need 10-seconds of motion as opposed to 10-seconds of light :-)