Perhaps the most apparent is to increase precision, which is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the guts distance of the tooth mesh. Sound can be affected by gear and housing components along with lubricants. In general, be prepared to pay more for quieter, smoother gears.
Don’t make the error of over-specifying the electric motor. Remember, the insight pinion on the planetary should be able handle the motor’s output torque. Also, if you’re using a 
multi-stage gearhead, the output stage must be strong enough to soak up the developed torque. Certainly, using a more powerful motor than required will require a bigger and more expensive gearhead.
Consider current limiting to safely impose limits on gearbox size. With servomotors, result torque is definitely a linear function of current. So besides protecting the gearbox, current limiting also shields the engine and drive by clipping peak torque, which may be from 2.5 to 3.5 times continuous torque.
In each planetary stage, five gears are at the same time in mesh. Although you can’t really totally get rid of noise from such an assembly, there are several ways to reduce it.
As an ancillary benefit, the geometry of planetaries fits the form of electric motors. Thus the gearhead can be close in diameter to the servomotor, with the result shaft in-line.
Highly rigid (servo grade) gearheads are generally more expensive than lighter duty types. However, for quick acceleration and deceleration, a servo-grade gearhead could be the only wise choice. In this kind of applications, the gearhead could be viewed as a mechanical spring. The torsional deflection resulting from the spring action increases backlash, compounding the consequences of free shaft motion.
Servo-grade gearheads incorporate several construction features to reduce torsional stress and deflection. Among the more prevalent are large diameter result shafts and beefed up low backlash gearbox support for satellite-equipment shafts. Stiff or “rigid” gearheads have a tendency to be the costliest of planetaries.
The type of bearings supporting the output shaft depends on the strain. High radial or axial loads generally necessitate rolling element bearings. Small planetaries could get by with low-price sleeve bearings or other economical types with fairly low axial and radial load capability. For bigger and servo-grade gearheads, durable result shaft bearings are often required.
Like the majority of gears, planetaries make noise. And the quicker they operate, the louder they get.
Low-backlash planetary gears are also available in lower ratios. While some types of gears are generally limited to about 50:1 or more, planetary gearheads prolong from 3:1 (one stage) to 175:1 or more, depending on the number of stages.