When a thread has nowhere to go on the current topic, but the new topic is valid. The post can be closed and the original poster informed with a recommendation of starting a new thread to follow the new topic. The posting of this change in the thread is optional, and to save questions later is often done. But it is up to the staff to decide if posting this info is necessary, or preferable. Also sometimes this action can be requested by the original poster, and is granted by a mod.
Bill, in my experiencing when a thread wanders off course, the mods will typically bring it back in line, if that's standard practice. Most forums let 'em run whatever direction they take. Going off course alone wouldn't be a reason to close the thread. The thread would need to take a real nose dive into the negative zone.
I agree with you often people repeat threads/subject matter which be a reason to close or combine a thread, but there 8is typically an indication why is was closed or combined.