Bug Watcher allows you to watch entire JIRA projects instead of individual issues.
Example Use Case
You want to get notified of changes made to all issues of a project. JIRA Notification Scheme in this project specifies the events Watchers should be notified.
Instead of setting yourself a watcher of all issues in the project, simply start watching the project via the Watches project tab.
As a result you will receive emails on all issues of the project about issue events set for watchers in the notification scheme.
Manage Project Watchers
Just as if you did for issues, you can (if you're authorized to) add or remove project watchers easily.
Example Use Case
Your project admin assigned the Manage Watchers permission to you so that you can add or remove watchers to or from an issue.
You want to make sure that your users watch all issues of a project.
Instead of adding them as a watcher to all issues, you add them as project watchers.
As a result, you users will receive notifications as if they were set as a watcher of all issues of a project.
Note, the users can watch/unwatch a project themselves.