A developer often has several project hierarchies, each with a different purpose. Some projects are used for Insulated Development, others for Collaborative Development. If a developer is also a team lead, some of those projects might be used for Integration Testing. Usually, a team lead builds a few projects, but occasionally another must be built. For example, if developers cannot build their projects in Collaborative Development, the team lead might move the projects to the Insulated Development project grouping. The team lead then can build the projects in a more controlled environment.
Think of adding a project to a project grouping as moving a different project into the workspace you are currently developing in. Instead of making another copy of the project, you can move the project to a different project grouping, and then update the project.