In SketchUp, importing preexisting terrain is your easiest route to creating terrain. However, the tools for creating terrain from scratch are useful in the following scenarios:
Terrain is important to many SketchUp modelers: Your building needs ground to stand on, or maybe you're modeling the ground itself to create a landscape. But wait. SketchUp's Sandbox tools - the tools you use to model terrain - can also create forms completely unrelated to terrain. How can terrain include all these other possibilities?