What is the difference between a method returning a task and an async method returning a task?

Question

I am trying to see what difference does adding async to a method which returns a task make.

Method in controller:

 public async Task<IHttpActionResult> GetSomeData()
    {
        var result = await _myService.GetAString();            

        return Ok(result);
    }

In MyService:

public Task<String> GetAString()
{
    return _repository.GetAStringAsync();
}

In repository:

public async Task<String>GetAStringAsync()
    {
        return await ......configureawait(false)
    }

Adding async await to the Service method works just fine. But what is the difference between these two service methods then? The only difference I can spot is that using version 2 I can use configureAwait(false)

**In MyService: ( cannot use configureawait)**


public Task<String> GetAString()
        {
            return _repository.GetAStringAsync();
        }

And
In MyService: (can use configure await)

public async Task<String> GetAString()
        {
            return await _repository.GetAStringAsync();
        }

Thanks