Guys tend to favor either barbells or dumbbells in their training. Knowing when and why you should use them will lead to the best payoff. Here's a breakdown of the advantages to each.
Better for beginners. Having to stabilize the weight you're holding in each hand improves coordination and activates more muscle fibers over all.
Help to even out strength imbalances between limbs (this is a problem especially common in beginners).
Safer than a barbell. If you can't complete a rep, you can drop the weights. You won't get pinned under them during a bench press or squat like you would a bar.
Allow you to go much heavier. Because of this, barbell exercises should always be done before dumbbell moves.
Better for working legs. You can do squats and deadlifts with dumbbells, but eventually the weight you use will get too heavy or cumbersome to hold. A barbell lets you use a big weight more comfortably.
Necessary for explosive exercises such as power cleans and snatches. You won't be able to generate the same power with a dumbbell.