Many GIS problems can only be solved using 3D. Seeing your data in 3D can very quickly highlight spatial relationships between GIS features, and analytical tools can quantify these relationships into patterns.
Environmentalists, forestry departments, and civil engineers can use 3D Analyst to understand and sculpt terrain to allow for events such as water runoff and flooding. Mining companies, geologists, and researchers can use 3D Analyst capabilities to learn more about subsurface geological bodies, such as the 3D intersections of deviated boreholes and subsurface strata. Local governments, city planners, and military organizations can leverage 3D Analyst to ask complex 3D questions about man-made structures, especially in regard to both current and proposed lines of sight within an urban area.