Why does CAD matter when you are choosing a ring? It lets you see a design before any metal is used. CAD software can create photorealistic images, called renders, of a piece before it is made 1. A jeweller can check a design this way without the cost of making each version 6.
How does it work? A CAD model is a "virtual solid". It can be turned, viewed from any angle, built up or cut away, and even weighed before it is made. CAD files can also be saved, copied and reopened years later to make changes 1. Designers build the model to scale and within limits such as size, style, cost and the way it will be made 2.
What happens after the design is finished? The CAD file can be sent to CAM (computer-aided manufacturing) hardware, such as a 3D printer, which turns the digital design into a physical model 3. Models made this way can be used for casting, which means pouring molten metal into a mould shaped from the model 4. You can read more in how the custom jewellery process works.
Example: Say you like a solitaire ring (a ring with one centre stone) but want a slimmer band. The jeweller changes the band in the CAD model, shows you a new render from every angle, and only sends the file on to be made once you are happy with it.