The Expression in this tutorial could be applied in more situations than I could list...usually as a small part of a lot of Expressions and constraints. Off the top of my head, I know it has been used in things like the Hand Gizmo and the face rig in the Squetch Rig...it's a building block for a lot of setups. It's similar to an "orient like" constraint, but the rotation is able to be isolated on a per axis basis and requires actual rotation being applied to the source bone.
Expressions in general are extremely powerful...this tutorial is a very simple example. They are a big part of a rigging toolkit along with constraints, Smartskin and bone parenting...each used for different reasons depending on the situation.
Hope that is helpful, Jason.
Thanks again