Preview function during live view is useful for focusing and framing adjustment when shutter speed is set to "Bulb", “Time” or a longer time than 30 seconds.
In astrophotography, the darkness of the subjects implies that the live view image is also too dark, making confirmation of your composition difficult.
Exposure preview helps focusing and framing with a brighter preview image, however, D-SLRs other than the D810A do not offer this function in Bulb and Time.
The D810A displays a virtual image preview that is equivalent to that obtained at 30 seconds,
when the shutter speed is set to longer than 30 seconds, allowing easy focusing and composition
(this exposure may not match that of the resulting image).