This is an innovation by the 2Bits company. They figured sending out email for node updates should not occur instantly, but rather once per hour via cron. Job Queue is a "garbage collector" that disposes on its jobs on cron runs. The Queue mail module catches email that are supposed to be submit through Drupal. And combine that with the Elysia Cron module and you'll find yourself with the best Mail-Job-Cron system open source can buy.
Note: you don't need to use this, if your site hardly sends out emails.
- Efficient resource usage
- Effect mail queueing and processing