The night of the rehearsal dinner is a combination of boot camp and family reunion. The actual wedding rehearsal serves as the boot camp portion of the evening because it's the last and only opportunity for instructing all the players in the ceremony about their various roles. The dinner that follows the rehearsal is often more relaxed. It allows the Bride and Groom to spend time with their inner circle and to begin the celebration of their impending nuptials. The Best Man and Groomsmen will play an integral part in this night. It's where we start to separate the men from the boys.
The Goal of the Rehearsal Dinner
Your primary goal is to learn your ceremony and wedding reception duties. Your secondary goal is to enjoy your time with the Bride and Groom and their families before the big day.
Why Have a Rehearsal Dinner, Anyway?
When I first became involved with the wedding scene, I could never understand why everyone took the rehearsal dinner so seriously. What was the big deal? Invite some people over, say some nice words, and have some food and drink. So why do we have to practice, and why is everyone so uptight about all the small details?
This cavalier attitude dramatically altered during my own wedding.
As a couple, you end up talking about your wedding day for six months to a year, and at some point you can't remember what else the two of you used to talk about or do with your time. You make more decisions in that time than you made in your life, and you hope they add up to a day your family and friends will enjoy. It turns out that the rehearsal dinner is one thing that helps minimize the stress by helping to reduce the chances of a total wedding day failure. Now I know why armies train so much-there's nothing like a real-time practice session to get everyone on the same page. If your response to this is that all weddings are the same, it's time to look closer. Although all weddings have the same goal, very few get there exactly the same way. 
More on the rehearsal? Read this article on The Rehearsal.
