Great product designers possess one unique ability: they can visualise the object they plan to build in three dimensions. In fact, they can usually visualise all the parts of the device working in concert while mentally subjecting the device to different test environments.
As an example, I have designed a number of boats and small ships in my day. These vessels have propulsion systems, steering systems, pumping systems, communication systems and alarm systems. To fully design one of these boats, you have to imagine each modification as it performs under different but likely circumstances.
Say you are designing a bilge pumping system. You will want to visualise pumping a bilge in the middle of a raging storm while the boat is taking on water. You must imagine trying to operate the pump and valves while standing on an oil-covered deck pitching violently in all directions. How well you envision different scenarios determines how usable the eventual design ends up.
While there is no software that does what these designing minds can do, there is software that illustrates an envisioned product so builders can fabricate what the designer has in mind.
Unfortunately, these mechanical computer-aided design (MCAD) applications are incredibly expensive and difficult to master because they were created for mechanical engineers (not designers) for production and milling machine interface requirements.
But this is much more than any product designer really needs. Designers need precision but, after they have designed their widgets, they need only to export the designs to another MCAD application.