Superman gave James Olsen the watch just in case he ever got into trouble, so the latter would be able to contact him. James is also able to use the watch to communicate with Supergirl when she is in the field.
At some point, Kara gave Alex a signal watch for in case her sister ever needed help. Kara and Alex also arranged a special code between them for whenever the latter uses the signal watch: pressing it once means that Supergirl should come; pressing it twice means that she shouldn't come, because it's a trap.
After Lena Luthor discovered her secret identity, Kara gave her a signal watch to use in emergencies.
Behind the scenes
- Jimmy Olsen's signal watch was first featured in Action Comics #238 (March, 1958) and was created by Otto Binder and Wayne Boring.