You know what your thing should look like, and what it should do. It is probably something you have come to know like a good friend. You have probably spent hours of creative time working on it, thinking about it, and revising it over and over again. “Obsession” is a word that usually describes it. That is okay. We understand. 🙂
Most folks need to sell their thing or idea someday. The people who need to visualize that thing are usually investors or users who would very much like to see and understand your thing before investing their resources in it. Sometimes it is not financially practical to have a complete prototype ready to show to investors, especially when those investors are not located where you are. Consider too that to see a photo and to have an actual prototype in hand present different risks for having your idea copied by others.
Concept drawings of the Adrenaline Mount http://www.adrenalinemount.com/
One of the strengths of designing parts or complete assemblies in 3D CAD software is that we can generate photo-realistic images of what the part will look like when it is made. We can place logos, add photo-realistic lighting, backgrounds, or whatever is needed to depict the device. Sometimes those images lead to aesthetic changes that can mean all the difference in the final product. Sometimes those images get used for promotional materials that would be difficult to generate by using a physical model. Sometimes those images go no further than a portfolio.
Sometimes a photo is indeed worth a thousand words.