Always use the 'async' and 'await' keywords in asynchronous methods in a library?

Summary: In a library method, when should I use the async and await keywords instead of returning a Task directly?

I believe my question is related to this one. However, that question is about .NET 4.0 and the TPL, while I'm using .NET 4.6 with the async and await keywords. So, I think my question might get different answers because these keywords didn't exist when the linked question was answered.

Explanation: I'm writing a simple wrapper for an external WCF service and the wrapper makes multiple SendAsync calls. Now I think that each wrapper method should just return a Task<> directly without being awaited. My understanding is that async/await should be used on the application layer, and not within a library.

So, for example, here is the approach that I think I should take for each wrapper method:

private Task<SignResponse> GetSignDataAsync(SigningRequestType request)
{
    return _service.SendAsync(request);
}

But on the Internet, I found several posts that use this approach instead:

private async Task<SignResponse> GetSignDataAsync(SigningRequestType request)
{
    return await _service.SendAsync(request).ConfigureAwait(false);
}

And here is another example that I found on technet:

async Task PutTaskDelay()
{
    await Task.Delay(5000);
} 

private async void btnTaskDelay_Click(object sender, EventArgs e)
{
    await PutTaskDelay();
    MessageBox.Show("I am back");
}

So, when should I use the second approach (the one that includes the async and await keywords)? Why not just return a whole Task without making PutTaskDelay async? I think that I should return Task directly whenever it is possible, and use async/await to get a final result in the application layer only. Am I right? If not, what is the difference between the two approaches that I show here?

My concern: When the async and await keywords are used, it seems that it just provides additional work to the compiler without any benefit.