Justification

C would have been simpler without incomplete types. However, they are necessary for void, and there is one feature provided by incomplete types that C has no other way to handle, and that has to do with forward references to structures and unions. If one has two structures that need pointers to each other, the only way to do so (without resorting to potentially invalid casts) is with incomplete types:

   struct a { struct b bp; };
   struct b { struct a ap; };