In C++, what's the benefit of having a class with functions...

say

class someClass{
 public:
  void someFunc(int arg1);
};

then having the function's actual functionality declared after int main

int main() 
    { return 0; }

void someClass::someFunc(int arg1)
    {   cout<<arg1; }

Furthermore, what's the benefit of declaring the class in a .h header file, then putting the functionality in a .cpp file that #includes the .h file?