Linked from
The 13 pages that link to Vertical datum, each with the reason it gives.
BathymetryRelated: Depth values are meaningful only in relation to a stated reference surface.
ElevationRelated: It specifies the zero surface from which an elevation is measured.
Digital elevation modelRelated: The datum determines what zero elevation means in a model.
Sea levelRelated: Elevation depends on the datum chosen to represent sea level.