Why the data type int is passed as a parameter in the operator function?

#include<iostream.h>
#include<conio.h>

class x
{
      private:
              int a;
      public:
             x(int j)
             {
                   a=j;
             }
             x operator++(int)        //postfix 
             {
                    return(a++);
             }
             void display()
             {
                  cout<<a;
             }
};

int main()
{
    x o1(10);
    cout<<"Before increment :";
    o1.display();
    o1++;
    cout<<endl;
    cout<<"After increment :";
    o1.display();
    getch();
    return 0;
}