With improvements in materials and control that produce consistant results, 3D Printing can now offer options for custom gears and other power transmission that previously would require expensive injection moulding procceses. The initial investment is lower and the turn around is quicker, you can prototype with and go to production quicker.
The unique geometry options with 3D Printing also offers lighter options and the so called impossible geometry that normal production methods simply could not or price of production would be prohibitive.