SubControllers are MVC Controllers that are also parameters to your action methods.  Incorporating their use in large systems allows for composition, dependency inversion, and separation of concerns.
Sometimes we want Controller actions to pass objects into SubControllers so that the SubControllers can do interesting things with them. 
For example, in a Product Controller we have the [...]

Eilon was correct - they made it easier!
It's trivial to test TempData now. This is all that's needed to port the example from his earlier post to Preview 4 (thanks Ben):
[TestFixture]
public class HomeControllerTests
{
[Test]
public void IndexSavesUserIDToTempDataAndRedirects()
{
var [...]

Dan North, in a recent article about DRY tests, says:
[Tests] are the documentation narrative that will guide future programmers (including yourself when you come back to change this code in three months time and you’ve forgotten what it does). In this case, clarity of intent is found in the quality of the narrative, not necessarily [...]