I am mechanical engineer and I use parametric design programs since 2010.
These programs allows the precise realization of 3D projects, both individual parts and assemblies, as well as the associative generation, based on the 3D models of the 2D execution and assembly drawings.
Its are called "parametric design programs" because at any time you modify a previously defined size or property (parameter), automatic whole modeled body changes.
Programs are used extensively in industry, in the design. The resulted files can be exported in different formats (.stl, .igs, .stp or other) that can be read by machine tools with numerical control and thus can be obtained physical parts.
I also use dedicated software for FEA, which means that I can virtually test the 3D model, in order to find out if the real object will resists in exploitation.