The design/build process is not a new or radically different concept. It simply refers to an end project being designed and manufactured by the same team. Who better to build a product than the people who designed the original plan, and vice versa?
Take a Boeing jetliner, for example. The manufacturers for these commercial aircrafts are completely responsible for the design, engineering, construction, and testing of the planes before they are deemed suitable for passengers. When you fly on a commercial plane, you are trusting that Boeing has assembled a team of expert designers, engineers, and manufacturers to work together to plan and construct a safe and reliable airplane.
This in-house design/build process works well for aircraft engineering, and it works great for developing real estate as well. You, the client, are in need of a reliable and high-quality product, a building, and you want to reach a completed project with as little hassle and cost as possible.
The way to eliminate hassle, stress, cost, and unexpected problems, is to reduce the number of people involved in the equation. The fewer people that you need to interact with, the fewer chances there are for errors and complications due to miscommunication. As your sole source provider of design and construction services, you have the convenience of only needing to communicate and coordinate with one person in order to bring your plans to fruition.