Applications of finite state transducer in software design

I have some understanding of FSMs, and the role of state machines in software design.

However, I can't wrap my head around finite state transducers. I know they are used in natural language processing a lot. However, there's no good example of a general purpose application of FSTs in software design.

What are some example applications of finite state transducer? Why isn't it a more common design pattern?