Many top bodybuilders have recently spoken out about their preference for alternating nearly "all-barbell" and "all-dumbbell" days. They believe that training in this manner, they are able to isolate certain muscle fibers and stabilizer muscles, which can often become overlooked using the same exercises in the same order each week. Let's look at a standard example of how chest day could be trained in consecutive weeks, alternating barbell and dumbbell day.
Workout #1 - Barbell Day
Workout #2 - Dumbbell Day
As you can see, entirely different exercises are used. The chest, the targeted muscle group of both of these routines, would be hit completely different on workout #1 than it would be in workout #2. Different angles, as well as the recruitment of stabilizer muscles (workout 2) and secondary muscle groups (emphasized in workout 1) also come into play.
The goal of this type of training is to hit all the available muscle fibers (fast twitch being hit by low reps, and slow-twitch being hit by high-rep sets). In addition, the stabilizer muscles are isolated on movements requiring dumbbells, which require additional balance from the lifter. Remember that the weight is pulled by gravity differently on dumbbells than it is for barbells.
Taking it a step further, it's entirely possible to utilize this "absolute" method in training other body parts are well.
If you're looking for some variation in your workout routine, try alternating dumbbell and barbell days. You might find your overall lifts on the compound movements might increase, as your new angles of approach to the muscle groups will cause small gaps in musculature to be filled in!