This listener will add the current user as a watcher to the issue. This is useful for instance if you want all commenters to be added as watchers.
It is similar to using the Participants field in the notification scheme, however using this listener allows people to remove themselves from the watchers.
For example, to add all commenters as watchers add the listener, select the project, and choose the Issue Commented event. You can further refine this by using a condition, for instance if you wanted to restrict this behaviour to just certain components or certain priorities of issue.