Use the Displacement Modifier if you want to have particular settings for that use of displacement - e.g. for an object that goes behind the camera but is still seen in reflections so needs to have the same quality of displacement even when out of the camera frustum, then you can use the Displacement Modifier so that object is set to World, leaving everything else in the scene set to the more memory friendly Screen. Or if you want to raise the displacement quality on one particular use (by having a smaller Screen value for just that object). That's the main use for it.