Real world example of getting value from std::future before std::thread::join is called

Question

I start learning about std::thread and std::future, and wrote very trivial example (I wanted to get the value from std::future before std::thread ends):

#include <iostream>
#include <future>
#include <thread>
#include <chrono>

int foo(int a, int b)
{
    return a + b;
}

int main()
{
    auto task = std::packaged_task<int (int, int)>{foo};
    auto future = task.get_future();
    
    auto t = std::thread{
        [&task](int a, int b)
        {
            using namespace std::chrono_literals;
    
            task(a, b);
    
            std::this_thread::sleep_for(5s); // something heavy to do
            std::cout << "Thread ends.\n";
        }, 1, 2
    };
    
    std::cout << future.get() << '\n';
    std::cout << "Waiting for thread\n";
    t.join();
    
    return 0;
}

Fisrtly, I wonder, if a real world example may be something like: logging, or saving something into database.

And secondly, if I cut out the std::this_thread::sleep_for(5s), is there any point of using std::thread instead of std::async?