Perhaps the most apparent is to improve precision, which really 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 and also lubricants. In general, expect to pay more for quieter, smoother gears.
Don’t make the error of over-specifying the engine. Remember, the insight pinion on the planetary should be able deal with the motor’s result 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 necessary will require a larger and more expensive gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, result torque can be a linear function of current. So besides protecting the gearbox, current limiting also protects 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 eliminate noise from this assembly, there are several ways to reduce it.
As an ancillary benefit, the geometry of planetaries matches the shape of electric motors. Therefore 
the gearhead can be close in diameter to the servomotor, with the output shaft in-line.
Highly rigid (servo grade) gearheads are generally more expensive than lighter duty types. However, for speedy acceleration and deceleration, a servo-grade gearhead may be the only wise choice. In this kind of applications, the gearhead may be viewed as a mechanical springtime. The torsional deflection resulting from the spring action increases backlash, compounding the effects of free shaft motion.
Servo-grade gearheads incorporate several construction features to reduce torsional stress and deflection. Among the more common are large diameter output shafts and beefed up support for satellite-gear shafts. Stiff or “rigid” gearheads tend to be the most costly of planetaries.
The kind of bearings supporting the output shaft depends on the load. High radial or axial loads generally necessitate rolling component bearings. Small planetaries could get by with low-cost sleeve bearings or additional economical types with fairly low axial and radial load capacity. For larger and servo-grade gearheads, heavy duty result shaft bearings are often required.
Like most gears, planetaries make noise. And the quicker they operate, the louder they obtain.
Low-backlash planetary gears are also available in lower low backlash gearbox ratios. While some types of gears are generally limited by about 50:1 or more, planetary gearheads expand from 3:1 (solitary stage) to 175:1 or even more, depending on the number of stages.