std::promise set_value and thread safety

I'm a bit confused about the requirements in terms of thread-safety placed on std::promise::set_value().

The standard says:

Effects: Atomically stores the value r in the shared state and makes that state ready

However, it also says that promise::set_value() can only be used to set a value once. If it is called multiple times, a std::future_error is thrown. So you can only set the value of a promise once.

And indeed, just about every tutorial, online code sample, or actual use case for std::promise involves a communication channel between 2 threads, where one thread calls std::future::get(), and the other thread calls std::promise::set_value().

I've never seen a use case where multiple threads might call std::promise::set_value(), and even if they did, all but one would cause a std::future_error exception to be thrown.

So why does the standard mandate that calls to std::promise::set_value() are atomic? What is the use case for calling std::promise::set_value() from multiple threads concurrently?