What is the purpose of AsQueryable()?

The most valid case I have for AsQueryable is unit testing. Say I have the following somewhat contrived example

public interface IWidgetRepository
{
   IQueryable<Widget> Retrieve();
} 

public class WidgetController
{
   public IWidgetRepository WidgetRepository {get; set;}


   public IQueryable<Widget> Get()
   {
      return WidgetRepository.Retrieve();
   }
}

and I want to write a unit test to make sure the controller passes back the results returned from the repository. It'd look something like this:

[TestMethod]
public void VerifyRepositoryOutputIsReturned()
{    
    var widget1 = new Widget();
    var widget2 = new Widget();
    var listOfWidgets = new List<Widget>() {widget1, widget2};
    var widgetRepository = new Mock<IWidgetRepository>();
    widgetRepository.Setup(r => r.Retrieve())
      .Returns(listOfWidgets.AsQueryable());
    var controller = new WidgetController();
    controller.WidgetRepository = widgetRepository.Object;

    var results = controller.Get();

    Assert.AreEqual(2, results.Count());
    Assert.IsTrue(results.Contains(widget1));
    Assert.IsTrue(results.Contains(widget2));
}

where really, all the AsQueryable() method allows me to do is satisfy the compiler when setting up a mock.

I'd be interested where this is used in application code though.

There are a few main uses.

1. As mentioned in other answers, you can use it to mock a queryable data source using an in-memory data source so that you can more easily test methods that will eventually be used on a non-enumerable based IQueryable.

2. You can write helper methods for manipulating collections that can apply to either in-memory sequences or external data sources. If you write your help methods to use IQueryable entirely you can just use AsQueryable on all enumerables to use them. This allows you to avoid writing two separate versions of very generalized helper methods.

3. It allows you to change the compile time type of a queryable to be an IQueryable, rather than some more derived type. In effect; you'd use it on an IQueryable at the same times that you'd use AsEnumerable on an IEnumerable. You might have an object that implements IQueryable but that also has an instance Select method. If that were the case, and you wanted to use the LINQ Select method, you'd need to change the compile time type of the object to IQueryable. You could just cast it, but by having an AsQueryable method you can take advantage of type inference. This is simply more convenient if the generic argument list is complex, and it is actually necessary if any of the generic arguments are anonymous types.