In spacewar is necessary to communicate between game components. For example game core have to notify player that he lost planet. Direct API calls can't be used at least because of cyclic dependencies.
Evens work is a same way as events in java swing API.
Following example show sequence of calls between user and components. When use change language in main menu than this event is passed to MainMenuController. MainMenuController raise new event that language was changed. Language event controller will trigger all registered listeners.