It lets you hold it in a few different ways, with your fingertips for work where you need to accurately feel what the tip is doing. With the palm of your hand when you need to apply lots of torque. It doesn't roll when you set it down. You can smack the end of it when you need to shock a slightly rusty or reluctant screw.
It's ironic because i think these are the cheapest design and i suspect most companies invent their own design precisely because this design is fairly evocative of old cheap mass produced hard plastic handles.
They just feel so good in your hand though. It's the perfect handle design.