Comment by duffmancd

It is much like a game might use a physics engine, or a new language might use the LLVM backend. To overly simplify, a CAD kernel will keep a list of operations (make a cube of this size here, drill a hole of this depth here, round these edges but not those). And combine that into a final volume. These responsibilities only get more and more complex as a part gets more complex - so using a pre-built engine allows CAD software to focus on tools and workflows to translate human instructions into lower-layer kernel geometry: the UI/UX. It also crosses into compatibility, if you use the same Kernel as another CAD it is much simpler to export/import from them. Otherwise you would have to reimplement their kernel (or enough of it), or be stuck exporting triangulated versions of the final volume - sort of like converting an image from vector to raster.