> My thinking was why require an intermediate processing step of the moc, if the core c++ is capable enough.

Sadly it is not. C++ does not provide reflection - not without macros duplicating all your function names. GTK+ does not provide reflection. For instance with Qt you can just define a C++ object and directly access its properties & methods from Javascript, which is extremely useful if you want to add a scripting layer to an existing software.

You can also iterate over all the properties of an object to create UIs automatically.