Part of the principle of "buildability" means that the 5x5 house is completely designed.* That means all the electrical circuits are drawn, and it means that all ductwork and plumbing is laid out with respect to structural framing. This process is known as "coordination," and usually happens much later in the building process, if at all. It has a number of benefits to you as a homeowner.
First, it allows you to understand the systems in your home. With knowledge, comes power. THE virtual model will allow you to look at any pipe or duct running in the crawl space and know what it is carrying and where it is going. If anything breaks, like a clogged pipe, for example, you can make a quicker diagnosis, find the nearest clean-out, and even run the snake yourself.
Second, it allows you to have meaningful, intelligent conversations with contractors when it comes to the difficulty of choosing among the myriad options available to you, say, with respect to HVAC systems. While the contractor will inevitably have recommendations on how they would prefer to install the system, the diagrams below provide a clear base from which to begin. Because of the technical nature of the building trades, homeowners are often left with no choice but to trust their contractors' recommendations. It should have to be this way. With a complete diagram in hand, and a little research on the internet, you should be able to have an intelligent, productive conversation with anyone working on your home.
Third, as mentioned before, coordinating the mechanical systems can save money. As mentioned before, often this coordination exercise never happens at all, which usually means a number of "change orders" when contractors have to come back to move things, or re-route things because something has been put "in their way." What this means to you is the risk of delays; and time is money. Better to model it virtually, and get it right the first time!
The structural system of the 5x5 house is also completely designed,* using generally valid rules to size the different members. While the plan set will need to be stamped by a structural engineer in your local jurisdiction, again, this nonetheless provides a solid base from which to begin. Any adjustments should be minor. This means builders can begin the process of pricing the project right away, without waiting to hear back from the engineers. It can't be said often enough: speed is everything, and the fewer tasks that must be done in sequence, the better.
* Structural, HVAC, Electrical and Plumbing Systems must be approved by licensed professionals in order to be compliant with particular building code regulations of the local jurisdiction. The drawings contained herein merely stipulate intent, and demonstrate feasibility.