Using an empty bar allows you to work through slightly more range of motion than an ab wheel. The bar sits a little lower to the ground which means your body can fall flatter.
This will also place the points of contact (the bar and your knees, or feet if you're doing them standing) further apart. The further the distance between these contact points, the harder your abs must work to keep your lumbo-pelvic region positioned correctly.
You've spent the spring and summer training to look good on the beach. As a result, you now have the chest and arms of a silverback gorilla, but the thighs and calves of a pink flamingo. Time to fix that.
Think of this as the Cliff Note's to the Ian King 101 class. Here you'll find the highlights of Ian's training principles that have changed the way most of us workout.