Drafting Lines vs. Modeling Data — the real difference was never 2D vs. 3D. It's whether a "wall" is a shape someone drew, or an object the model actually understands.
A 2D CAD drawing is, underneath, a collection of independent lines, arcs, and text with no inherent relationship to each other or to any real-world building object. A "wall" in a 2D CAD file isn't a wall at all — it's two parallel lines the drafter drew, that happen to look like a wall in this one view. Nothing in the file knows those two lines represent a single physical object, or that the same wall also needs to appear, correctly, in the section three sheets over and the elevation two sheets after that. A BIM model represents that same wall as one intelligent, parametric object — with real properties like height, material, and fire rating — that every plan, section, elevation, and schedule derived from the model can reference directly. The consequence of that difference shows up the moment anything changes.