I don't honestly see the purpose of using one of these. With the power steering I can turn the wheel with just the heel of my hand and I never need to hold it tightly. So I'm turning my wheel as if I have a knob, but don't.
Another reason is that like others have said, with the hydraulic steering it will usually always be anywhere but where you want it to be.
These things were invented before power steering and gave you extra leverage to turn the wheel. They were called suicide knobs because they could catch in clothing or break causing you to loose control of the vehicle in a turn.