I have an example from a current system I'm working on. I'm in the process of building a stock trading system. The process of tracking the state of an order can be complex, but if you build a state diagram for the life cycle of an order it makes applying new incoming transactions to the existing order much simpler. There are many fewer comparisons necessary in applying that transaction if you know from its current state that the new transaction can only be one of three things rather than one of 20 things. It makes the code much more efficient.
