From Wikipedia: Design–build is sometimes compared to the "master builder" approach, one of the oldest forms of construction procedure...
For us, the main benefit of the Design-Build methodology is the integration, rather than segregation, of the design phase and the building phase of a project.
Design Phase: By working with a team of experienced tradespersons early on in the design process, we have the ability to be creative and intelligent about the systems and details we incorporate into our designs. The collaboration also enables us to better manage our clients’ budgets and schedules, while minimizing conflict during construction.
Build Phase: The team that designs the project is the team that builds the project. Because we have designed the project, we have an intimate knowledge of the design and are able to address site conditions and client changes seamlessly, without losing sight of the design intention of the project. And because we have engaged our subcontractors during the design process, we minimize conflict during the project, allowing them to carry out their work efficiently and on budget.