Why would you declare an `IEnumerable<T>` `readonly`?

From the this [article on async & await](http://www.dotnetcurry.com/ShowArticle.aspx?ID=869) we have the following code.

```
class OrderHandler
{
    private readonly IEnumerable<Order> _orders;

    public OrderHandler()
    {
        // Set orders.
    }
    public IEnumerable<Order> GetAllOrders()
    {
        return _orders;
    }
}

```

`IEnumerable<T>` is immutable. How is this different from the `readonly` keyword?