std::ref (C++)

Why is this needed?

int yo = 5;
auto w = std::ref(yo);   // w is std::reference_wrapper<int>

#include <functional>
#include <iostream>
 
struct Counter {
    int value = 0;
    void inc() { ++value; }
};
 
int main() {
    Counter c;
 
    std::function<void()> f = std::bind(&Counter::inc, std::ref(c));
    f();
    f();
 
    std::cout << c.value << "\n"; // prints 2
}
 

#include <thread>
#include <iostream>
#include <functional> // std::ref
 
void add_one(int &x) {
    ++x;
}
 
int main() {
    int value = 0;
 
    std::thread t(add_one, std::ref(value)); // pass by reference
    t.join();
 
    std::cout << value << "\n"; // prints 1
}

#include <functional>
#include <iostream>
 
void scale(int &x, int factor) {
    x *= factor;
}
 
int main() {
    int v = 10;
 
    auto f = std::bind(scale, std::ref(v), 3);
    f(); // modifies v in place
 
    std::cout << v << "\n"; // prints 30
}