Autodesk Fusion360 is an amazing program, with CAD, Simulation, and CAM built into one program. The power of the modeling environment allows parts to be created that are tricky to machine without multiple setups. A setup could correspond to re-positioning the part in a vise. A common example is machining one side of a part, and then flipping the part to machine the second side.
For simulation to be most accurate and machining operations to be most efficiently generated, the stock of a setup must reflect the previous operations. Without doing this, and leaving the stock as a relative size box, collision detection is not as accurate and machining operations may be generated cutting material that no long exists.
In this video we learn how to export the results of a CAM simulation as an STL file. That STL file is then loaded as a mash and converted to a Brep solid model, superimposed on the part. The new solid model forms the stock of the subsequent setup. Of course, this method could also be used to import any STL file for other modeling purposes.