So why go with a softbox that is slower to set up, cost 5 times as much and breaks just as easily?
Heres an example.
Softbox on the left, umbrella on the right. Light going through an umbrella can come back into the lens and cause flare. You have be aware of your shooting angles and flash to subject distance.
Heres another example of light spill. That flair can rob contrast and sap color.
So why not use a softbox all the time and skip the umbrella? Well 28" vs 42" for starters. Yes, they make softboxes in all sizes, but not for 20 bucks! Umbrellas work great for lighting large groups of people as well as environments. Maybe you want more of the foreground lit. That's where umbrellas excel. They are like any other piece of kit, when used correctly they give a great result!
(side note-the light should have been placed lower in the image above. That would have given us a bit more light in the eyes. You can tell the position of the light from the shadow under her chin)
Now get out there and shoot! Just dont spill any light on ya!