A good design must be practical. The mold maker must be able to produce the components in a logical, orderly manner to make money. Often, close tolerance dimensions are specified when a much looser tolerance could have easily done the job.
Take an ejector pin plate, for example. Everyone knows that the thickness is basically irrelevant, but usually the dimension given is a close tolerance size. An experienced toolmaker will just ignore the tolerance and proceed, but nowadays, with the specialization of tasks in the shop, a less skilled operator would waste precious time holding an unreasonable tolerance.
The 3D geometry must be clean. The fast pace of mold making today makes it essential to have efficient, reliable software. The days of vague sketches, or toolmakers making up the design as they go are long gone. There are many excellent companies that offer high end software programs for designing molds, dies, and just about any kind of tooling you can imagine.
CNC machines need clean geometry to run properly. If the design is sloppy and the translation of different software messy, the end result will show it. Plus, the operator will have a much easier time running the programs with clean geometry. The design must be clear in it's function. It is maddening for a plastic injection mold maker to spend hours deciphering what the designer means. Information that is assumed or omitted can delay the construction by days and cause unnecessary errors. Why should a toolmaker spend time looking up information that was right in front of the designer at one time?
It is always much easier to include notes or details that show what is required than to search it out later on. Once the design is in process, and the information is available, why not simply give the mold maker the same information? For
example, a 3D drawing can visually clarify many questions.