Professional portrait shooters sure like using really big softboxes.
Notice how Joe has a assistant holding a softbox and moving the light to just the right spot on the subject while Joe has his camera set to exposure for the natural sunset light.
So why a softbox over an umbrella? Softboxes and umbrellas both soften light. The softbox creates a more defined edge to the light and is perfect for feathering the light on a model. How about umbrellas? They spread light more than a softbox and can be used as a bounced light source or a shoot-through source.