What is independent association actually doing here? What is its purpose?

Hi guys, 

I have been desperately trying to get my head around independent and FK associations. 

I have 2 models connected up with EF (I've left out few non-relevant properties): 

public class Item
{
    public int Id { get; set; }
    public string Name { get; set; }
    public Categories Category { get; set; }
    [Required]
    public int BoxId { get; set; }
    public virtual Box Box { get; set; }
}

public class Box
{
    public int Id { get; set; }
    public string Label { get; set; }
    public virtual ICollection<Item> Items { get; set; }
}

A box contains many items and so it's a 1..* relationship. When I create an item, I want a dropdown list to appear with all the available boxes. 

I have actually achieved this, but I am now trying to analyse and understand what EF is actually doing. 

For reference, here is my controller code:

public ActionResult Create()
{
    ViewBag.BoxesId = new SelectList(_db.Boxes, "Id", "Label");
    return View();
}

[HttpPost]
public ActionResult Create(Item item)
{
    if (ModelState.IsValid)
    {
        _db.Items.Add(item);
        _db.SaveChanges();
        return RedirectToAction("Details", new { id = item.Id });
    }
    ViewBag.BoxesId = new SelectList(_db.Boxes, "Id", "Label");
    return View(item);
}

And here is the dropdown menu:

<div class="form-group">
    @Html.LabelFor(model => model.BoxId, htmlAttributes: new { @class = "control-label col-md-2" })<div class="col-md-10">
        @Html.DropDownListFor(model => model.BoxId, ViewBag.BoxesId as SelectList, htmlAttributes: new { @class = "form-control" })
        @Html.ValidationMessageFor(model => model.BoxId, "", new { @class = "text-danger" })</div></div>

Now on submitting the form, my item property contains all the details I would expect, such as the box id, item.name etc. however, the actual Box box reference is null, which makes me wonder why is this even necessary? If I have the item and I want to find out the box, I can simply do a quick FK lookup and retrieve the box object. So why do I need the Box box independent association? 

I'm sure I am missing something and would be delighted if someone could hopefully explain to me the item's "Box box" reference purpose. 

I took a further stab in understanding this and I guess I could add something like below in the controller POST method, which would populate the box reference, but I doubt this is its intended purpose!

 item.Box = _db.Boxes.Find(item.BoxId); 

Thanks in advance!