3D modeling or CAD (Computer Aided Design) allows engineers and designers to build realistic computer models of parts and assemblies. These models can be then 3D Printed or CNC machined as well as used to run complex simulations.
Architects use their 2D plans to create 3D designs. They can then visualize and simulate their entire project using real-world parameters, without needing to build any physical structures or models. With CAD, architects are able to view different aspects and perspectives of their building from within a single program.