When should TaskCompletionSource be used?

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AFAIK, all it knows is that at some point, its SetResult or SetException method is being called to complete the Task<T> exposed through its Task property.

In other words, it acts as the producer for a Task<TResult> and its completion.

I saw here the example:

If I need a way to execute a Func<T> asynchronously and have a Task<T>
to represent that operation.

public static Task<T> RunAsync<T>(Func<T> function) 
{ 
    if (function == null) throw new ArgumentNullException(“function”); 
    var tcs = new TaskCompletionSource<T>(); 
    ThreadPool.QueueUserWorkItem(_ => 
    { 
        try 
        {  
            T result = function(); 
            tcs.SetResult(result);  
        } 
        catch(Exception exc) { tcs.SetException(exc); } 
    }); 
    return tcs.Task; 
}

Which could be used if I didn’t have Task.Factory.StartNew –
But I do have Task.Factory.StartNew.

Question:

Can someone please explain by example a scenario related directly to TaskCompletionSource
and not to a hypothetical situation in which I don’t have Task.Factory.StartNew?

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