One of the qualities of a good design is good documentation. An inventor may have invented a very good machine but if he is the only one who can operate it, then its useless. It does not truly serve the purpose. The same is true in design. The assumptions, references, calculations and all other data used one way or another in the design process to come up with the result should be well documented. This will be very useful in design verification. The verifier need to have a very good grasp of the basis of design in order to understand fully the design itself.
If the design does not have documentation, it may be that even the designer himself may not understand what he have done few months back. People are not computers where ther is a way to extract contents from its memory or storage. Unless put down in writing, other people could not read what the designer is thinking.
I can still recall, being a newly hired Engineer in one of the companies that I worked with, I went to a review meeting. I was wondering how the other Engineer marks up the design drawings. There are so many mark up which I believe should be referred to a calculation, design basis or specification. I then asked, what is your basis in commenting the drawing. He asnwered backed without any second thought. Its all in my mind. WHAT? I just said to myself.
Another example was when I was still working as a Service Manager with a company having an on-going maintenace contract with PRC. I was told that the Electrical Engineer was rude and very strict on workmanship. I asked then, was there any plans, specifications or documentation of the electrical works we have been doing for PRC, I was told none. The electrical engineer was just pointing to where he wants his cable ladder to be routed, lights to be installed and motors to be located. And if the result is not what he expected, then he will issue a rework. Much so, what is coming out of his mouth was rubbish. If only this engineer has a sense of responsiblity documenting what he wants even in a simple sketch, then other people will be able to understand.
These are examples where design documentations should have come in handy. In any design implementation, documentation should not be taken for granted may it be a small project or a very large one.