I know prefix operator doesn't store a copy it just modifies the instance but postfix operator stores a copy. Why? How come keeping a copy would be useful? What are the use cases? Here is a piece of code to illustrate my question.

#include <iostream>
#include <sstream>

class Int {
private:
    int x;
    std::string numberString;
public:
    Int(int _x):x(_x) {}
    Int operator   () {
      x  = 1;
      return *this;
    }
    Int operator   (int) {
      Int copy = *this;
        *this;
      return copy;
    }
    operator const char*() {
      std::ostringstream os;
      os << x;
      numberString = os.str();
      return numberString.c_str();
    }
};

int main() {
  Int a = 5;
  std::cout <<   a << std::endl; // 6
  Int b = a  ; // b is 6
  std::cout << b << std::endl; // 6
  std::cout << a << std::endl; // 7
}