ASP.NET and C# async

I'm confused about C# async. I understand async for parallel task processing. For example, a method does A and B task, and async enables A and B do action in the same time; B doesn't have to wait until A be done.

But the below example code does only a single task which is pulling data from a database. It means, there is nothing for parallel tasks. But why does it use async?

Please give me an advice so that I can understand async.

[ResponseType(typeof(BookDetailDTO))]
public async Task<IHttpActionResult> GetBook(int id)
{
    var book = await db.Books.Include(b => b.Author).Select(b =>
        new BookDetailDTO()
        {
            Id = b.Id,
            Title = b.Title,
            Year = b.Year,
            Price = b.Price,
            AuthorName = b.Author.Name,
            Genre = b.Genre
        }).SingleOrDefaultAsync(b => b.Id == id);
    if (book == null)
    {
        return NotFound();
    }

    return Ok(book);
}