C++ classes

What benefit does it have, if I write the prototypes of the functions my class will have and then later write the actual functions?

Like for example:

class foo
{
   public:
   int value;
   void getValue();
};

void foo::getValue()
{
  return value;
}

Couldn't I simply define the function inside the class?
Like:

class foo
{
  public:
  int value;
  void getValue()
  {
     return value;
  }
};

>>2

That does seem logical. But what difference does it actually make if I define the function outside of the class? If I want another implementation it's not a difference where I have to change the function, inside the class or some lines further down the code.
Or is it?