There’s an enormous difference between a modern CAD model and a Sketch document.
Pauli Olavi Ojala

“The CAD file is in an actual model that gets used as input to the next phase of the work pipeline.”

The same could be said of a prototype made with Sketch. It is an exact comparison of the CAD file. To say that it isn’t is saying that some part of the 3D digital file is actual used in the physical building.

Both the prototype and the CAD file are essential steps in the process, it just happens that the prototype of a UX resembles the finished product more closely which lends itself to more criticism of saying it’s an unnecessary step.