Well, for one, unions have defined behavior in C, while passing pointers around and casting them randomly doesn't. I don't know if this is the best example. Ge

Well, for one, unions have defined behavior in C, while passing pointers around and casting them randomly doesn't. I don't know if this is the best example. Generally you see unions in C to cover "tagged unions", I.e. Those things that would be subsumed by algebraic data types in other languages.