What is the point of .NET 4.6's Task.CompletedTask?

Task.FromResult(whatever) works for Task<TResult>, but until 4.6 there was nothing for the nongeneric task. You could use FromResult with a dummy value and implicitly cast it to Task, but that somewhat obfuscates the intent (you're not really returning any asynchronous value) and allocates objects underneath (whereas CompletedTask can be cached and shared between all callers).

It's not uncommon to see custom static completed tasks in current (4.5.2 and older) codebases, so in my opinion it makes sense to incorporate it to the framework itself.

Task.CompletedTask property is important when you need to give a caller a dummy Task (that doesn't return a value/result) that's already completed. This might be necessary to fulfill an "interface" contract or testing purposes.

Task.FromResult(data) also returns a dummy Task, but this time with data or a result. You probably would be doing this because you already have the data and have no need to perform any operation to get it.

Example Use

public Task DoSomethingAsync()
{
    return Task.CompletedTask; // null would throw exception on await
}

public Task<User> GetUserAsync()
{
    if(cachedUser != null) 
    {
        return Task.FromResult(cachedUser);
    }
    else
    {
        return GetUserFromDb();
    }
}