Some advanced Mac users like to have the Postfix email server locally. For mobile users, this allows your email client to always send mail and have Posfix handle delivery when back on a network.
For web developers, having a local mail server can assist with email sending in your development environment.
The Postfix Dashboard widget install comes with a command line program called postqueue which allows any user to see what mails are in the queue and to manually flush the queue.
Moreover the Postfix widget allows you to refresh to see the queue status and flush the queue with a click of a button.