Void is surprisingly useful.

Also note that C's rules around void are too irregular to make it work as a unit type. You can't declare a void variable. It's not really an empty type or a unit type, it's a secret third thing.

In C++ this forces templates to special-case void. There was a proposal to make void more regular, but that wasn't going to happen without breakage, so C++ added std::monostate instead, which is actually intended as a unit type.