Grip along the axis of the dart provides many benefits to the grip.
Primarily you're gaining radial positioning of your finger on the dart. This helps indicate your fingers consistently around the barrel. You can fine tune the number of grooves to better indicate your fingers. For example if you indicate your fingers 180 degrees from each other 6 grooves give you a perfect 12 and 6 o'clock positioning to consistently grab the dart.
This grip style be used to create a pixel like grip, create tear drop shapes, create start and stop 'morse code' style grip. Anything you can think of that adds grip along the length of the barrel.