Для чего нужны callback-функции

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
 
struct Employee {
    int age;
    int salary;
    std::string firstName;
    std::string lastName;
    std::string phone;
 
    friend std::ostream &operator<<(std::ostream &output, Employee const &emp) {
        output << emp.firstName << ' ' << emp.lastName
               << " (" << emp.age << "): " << emp.phone
               << " [$" << emp.salary << "].";
        return output;
    }
};
 
namespace cyberforum::sourcerer::utilities {
template <typename Container>
void print(Container const &container, std::string const &message) {
    std::cout << message;
    for (auto const &elem : container) {
        std::cout << elem << '\n';
    }
    std::cout << '\n';
}
} // namespace
 
int main()
{
    std::vector<Employee> staff {
        {20, 50000, "Igor", "Sokolov", "8908-234-34-34"},
        {34, 45000, "Oleg", "Knyazev", "8905-456-56-56"},
        {30, 48000, "Lada", "Volkova", "8906-123-23-23"},
        {29, 60000, "Vlad", "Petelin", "8908-808-08-08"},
        {42, 35000, "Stas", "Vronsky", "8907-227-07-27"},
        {31, 55000, "Vera", "Gromova", "8909-449-39-59"},
        {33, 37000, "Gleb", "Koshkin", "8904-444-54-64"},
        {23, 42000, "Seva", "Boyanov", "8909-129-39-49"},
        {43, 32000, "Dara", "Kamneva", "8907-567-77-77"},
    };
 
    using Emp = Employee const &;
    namespace rng = std::ranges;
    namespace utl = cyberforum::sourcerer::utilities;
 
    // 1
    auto byAgeInAscendingOrder = [](Emp a, Emp b) {return a.age < b.age; };
    rng::sort(staff, byAgeInAscendingOrder);
    utl::print(staff, "Sorted by age in ascending order:\n");
 
    // 2
    auto byAgeInDescendingOrder = [](Emp a, Emp b) {return a.age > b.age; };
    rng::sort(staff, byAgeInDescendingOrder);
    utl::print(staff, "Sorted by age in descending order:\n");
 
    // 3
    auto bySalaryInAscendingOrder = [](Emp a, Emp b) {return a.salary < b.salary; };
    rng::sort(staff, bySalaryInAscendingOrder);
    utl::print(staff, "Sorted by salary in ascending order:\n");
 
    // 4
    auto bySalaryInDescendingOrder = [](Emp a, Emp b) {return a.salary > b.salary; };
    rng::sort(staff, bySalaryInDescendingOrder);
    utl::print(staff, "Sorted by salary in descending order:\n");
 
    // 5
    auto byFirstNameInAscendingOrder = [](Emp a, Emp b) {return a.firstName < b.firstName; };
    rng::sort(staff, byFirstNameInAscendingOrder);
    utl::print(staff, "Sorted by first name in ascending order:\n");
 
    // 6
    auto byFirstNameInDescendingOrder = [](Emp a, Emp b) {return a.firstName > b.firstName; };
    rng::sort(staff, byFirstNameInDescendingOrder);
    utl::print(staff, "Sorted by first name in descending order:\n");
 
    // 7
    auto byLastNameInAscendingOrder = [](Emp a, Emp b) {return a.lastName < b.lastName; };
    rng::sort(staff, byLastNameInAscendingOrder);
    utl::print(staff, "Sorted by last name in ascending order:\n");
 
    // 8
    auto byLastNameInDescendingOrder = [](Emp a, Emp b) {return a.lastName > b.lastName; };
    rng::sort(staff, byLastNameInDescendingOrder);
    utl::print(staff, "Sorted by last name in descending order:\n");
 
    // 9
    auto byPhoneInAscendingOrder = [](Emp a, Emp b) {return a.phone < b.phone; };
    rng::sort(staff, byPhoneInAscendingOrder);
    utl::print(staff, "Sorted by phone in ascending order:\n");
 
    // 10
    auto byPhoneInDescendingOrder = [](Emp a, Emp b) {return a.phone > b.phone; };
    rng::sort(staff, byPhoneInDescendingOrder);
    utl::print(staff, "Sorted by phone in descending order:\n");
}