I think the best use case for KC3 is rapid prototyping in a high level language and progressive re-implementation in C, function for function.
In KC3 each function can be either a KC3 function or a C function. This way you can write your logic in KC3 and then port the functions one by one to a C module that you can load with dlopen(). The C functions are called using libffi using the Cfn module.