After the PCB design is ready to be tested, severeal PCBs are produced and assembled. The circuits and chosen components can be tested with the software as a system, and tried into 3D printed enclosures.
3D printed prototype enclosures are very good for ruling out any oversights and inaccuracies that can develop from virtual design flaws or information inaccuracies. Vendors might provide other sized PCB components than expected, or the 3D representation of a component could be faulty, so these enclosures are a very good place to rule out the last problems before large sums of money is spent on expensive production tools and making assembly lines.
Throughout the majority of hardware and software develomplent phases 3D printed enclosures are used for hands-on representation of the final product, user experience testing, and the convenience of operability.