Why do people want degree graduates? Because it shows they can finish things, because it shows they are smart? because it shows they have drive? Many reason are given for hiring grads.

Somebody with a PhD in comp sci would have pretty much proven they are good with very difficult problems and have the drive to see things through to completion.

What qualities do you think it takes to finish a PhD? Some employers seek out those qualities and a PhD is a good indication that you have those qualities.