Before we discuss how to change from 2D to 3D, it's important to understand why 3D is necessary. There are many reasons:
Our world is of course 3D, so humans naturally are able to interact with a map in 3D in a more natural way than in 2D. 3D is more realistic than 2D, so it makes understanding a design or analytical output easier.
For some applications of GIS, a 2D map is simply insufficient. If you're a military mission planner, 3D is just not a nice feature, it's a must. Imagine if you were trying to determine if an inbound flight had enough clearance to pass over some buildings. Without 3D, this would be impossible, so lives would be at risk.
3D allows urban planners to try out different designs before ever building anything. Using 3D allows the planner to see different possibilities to ensure other views aren't blocked or that a new building's shadow won't be cast on the other buildings using an artificial light source, which can be determined at different times of the year.
3D...