Well sending a message that a remote province is being invaded, is useful. Especially if you can send that message days faster than runners can send messages. I

Well sending a message that a remote province is being invaded, is useful. Especially if you can send that message days faster than runners can send messages. Its also very useful to coordinate ships. The difference between steam engines is that steam engines were very difficult to build. Semaphore is very simple and a trial system wouldn't be expensive to set up. The Romans did use crude fire signals, so I think that proves that they would find it useful.