1. There are crucial parameters only available in the 2D drawing.

Although some sorts of software start to offer an option that designers can input annotation and tolerance Etc. in 3D model, it is common to only annotate information like dimensions tolerance, hardness, surface treatment, geometric tolerance, material and other notes in 2D drawing. These parameters can mean huge differences in workflow and cost. For example, a 10mm diameter carbon steel shaft without a 2d drawing, the workflow is simple, CNC machine it and then deliver it. The same 10mm shaft with notes in 2D drawing saying: 45hrc, 10+/-0.01mm diameter, the workflow is CNC machining, heat treatment, grinding..and the cost is a lot more.

2. 2D drawings set up standards for quality control.

Dimensions can be taken from a 3D model, but a 3mm diameter get from a 3D model and a 3+/0.01mm annotation in a 2D drawing are very different in quality checking. Only with a 3D model, a 3.1mm diameter is a good quality, but with 3+/0.01mm annotation in a 2D drawing, that is an NG(not good) in the QC process.

So when you want the manufacturer to make parts for you, please try to provide 2D drawing with as much information as possible.