class member variable pointer? When do you use it?

#include <iostream>
using namespace std;

class Car
{
    public:
    int speed;
};

int main()
{
    int Car::*pSpeed = &Car::speed;

    Car c1;
    c1.speed = 1; // Direct access (typical)
    cout << "speed is" << c1.speed << endl; //1 output
    c1.*pSpeed = 2; // use pointer to member to approach (pass)
    cout << "speed is" << c1.speed << endl; //2 output
    return 0;
}