When is it needed/appropriate? And when not?

For example, should you create a conceptual mass model of a small house?

Not all building need to modeled using conceptual masses. Sometime the buildings are simple that you already have an idea of what it looks like and can complete it. It's appropriate when you want to do large scale designs like a business building or complex designs. When you're dealing with the size, it can be helpful to think of them in simple masses. But if you're designing a small building, you do not need conceptual masses. It could be helpful but you could be wasting time when you could already be building the walls, floors, etc.

Walt Disney Concert Hall (USA)

The Interlace (Singapore)

CCTV Headquarter (China)