What is this variable for? It doesn't seem to have any value unlike $this->getFrontController().

In what circumstances would I want to access the front controller from action controllers?

Zend_Controller_Action::$_frontController

This is a private member variable, and you won't really ever need to do anything with it. Its used internally by the class. Most of these store metadata of some kind. Use a debugger to step through the code if you're ever interested to see what kind of data it gets populated with.

Your action controllers will most likely never need to worry about the frontController, as it is just used to route the requests to your controllers, but never the other way round.

More info: P of EAA: Front Controller