3D objects in orthogonal projection sometimes look as if they were 2D. Perspective projection mode makes presentations look more realistic.
It is possible to switch between two modes using VTK viewer toolbar buttons:
Perspective mode button
Orthogonal mode button
For example, look at the tube shape in both views.
Shape in perspective mode
Shape in orthogonal mode
Further than that, it is possible to exercise full control of the presentation of a 3D scene by modification of camera attributes.
To understand how the scene is built, see this picture.
To change camera attributes, click Change View Parameters button in the VTK viewer toolbar.
The following dialog will appear: