What is the need for defining a method of a class, outside the class definition using the scope resolution operator?

Question

When it is possible to define a method of class within the class definition, I wonder why would we define a method outside of a class? What situations create a need for that?
Example :
class add
{
int a,b;
public:
add()
{ a='\0';
b='\0';
}

void display();
};

void add::display()
{
int sum;
sum = a+b;
cout<<sum;
}

Why would we define the method display() outside the definition of the class "add"?

Answer 1

Separating the interface and implementation of the class' methods in real-world software engineering is a must for having a well-organized and safe system.

[https://accu.org/index.php/journals/269]
[http://www.cppforschool.com/tutorial/separate-header-and-implementation-files.html]