What are the principal advantages of creating a conceptual design model using masses before diving into the detailed design and building element modeling?
The main purpose of starting with a conceptual design is to build a fundamental shape/aesthetic that will serve as a basis for designing smaller portions of the project. Owners also tend to care about the exterior look and overall movement through the design, "big picture", more than they do about the intricacies inside. All project stakeholders want to discuss space allocation up front, so the conceptual mass design serves as a quick solution to drawing up a draft of how the spaces will be divided up among the different usages.
Should all buildings be modeled first using conceptual masses?
Just large or unique buildings in particular, due to the complexity. Small or routine structures where the general aesthetic/movement through the building is already well understood/conventional can skip mass modeling.