Did you look at the link Delta posted? Explains it all right in the first post.
If you're running a generator, or in this case, a minifridge, or any auxilliary components that draw a lot of power, the second battery makes sure you can start your truck when you want to.
Perfect example: At a CART race several years ago, this bonehead with a stereoed up Suburban thought it would be cool to open all the doors and provide all of us with an acid rock and punk rock concert. Come time to leave, he couldn't start his truck!