What is cad?
CAD stands for computer-aided design. It has been around in some form or fashion for decades, originally developed as a way to increase productivity over a standard drafting board. When you say "CAD", you can be talking about a number of things...Architectural CAD (blueprints, etc), Electrical CAD (printed circuit board design, etc...referred to as ECAD), and of course Mechanical CAD (or MCAD), which is our realm...and the one generally referred to when talking about product design.
Except for very few instances, Mechanical CAD is the master set of data for a product. Once 3D part models are generated, 3D assemblies are built containing all physical design data for a product (including ECAD data, where applicable) - everything from the look and feel of it to the complete Bill of Materials.
3D models, however, are only half the equation. Though we no longer use drafting boards (don't worry though...almost all companies still employ draftsmen), there is still a need for 2D design data. In fact, though the 3D models are by far more interesting to look at, it is the 2D drawings which are truly the roadmap for how a part (and therefore product) is created. A good set of 2D drawings should allow any fab shop to produce parts with 100% of the original design intent, without having to ask questions.
Why do i need cad?
The truth is...almost EVERYTHING in the world today is designed in CAD. From airplanes to fidget spinners...there is a 3D model behind almost every physical product you see every day. And for good reason.
Though you can never get a complicated design 100% perfect in CAD without building a physical prototype ("It looked ok in the model!" is a phrase heard often by engineers)...you can get awfully close. What's even more important is you can iterate, iterate, iterate until the design looks just like you want it...all before spending a dime on fabricated parts. Moreover, once the work is done to put your design into a 3D model, you can make changes and build upon it very efficiently from there on out.
But what if your design is simple? Maybe you drew it on the back of a napkin and then made it with some simple tools around your shop. Well, it is possible you don't NEED to have your design captured in CAD. However, if you ever want to mass produce it, generate high quality marketing materials, or hand it off to a shop...you'll need to have the CAD data available. Which brings us to our next point: "Why do I need 2D drawings?"
The truth here, again...is you may not. If all you want to do is have parts 3D printed, there's no need for 2D drawings. The whole idea of "Rapid Prototyping" is the rapid part...and good drawings take time. However, your design data is only half complete with the 3D models. You really need to have a good set of 2D drawings before handing anything off to a fabrication shop. Though the industry is slowly moving towards capturing all design intent data within the 3D model, almost all shops will still need the drawings for: tolerances, material callout, finish criteria (anodize, powdercoat, chemfilm), any special machining callouts, weld callouts, and part inspection. There is an entire standard dedicated only to 2D drawings that we follow here at Trailride, which ensures we create drawings that can properly be interpreted by any good shop.
3D models get the idea out of your head and captured in a model you can see and interpret...2D drawings tell your suppliers exactly how to make it.