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 mistake of over-specifying the engine. Remember, the insight pinion on the planetary must be able handle the motor’s output torque. Also, if you’re utilizing a multi-stage gearhead, the output stage must be strong enough to soak up the developed torque. Obviously, using a more powerful motor than necessary will require a larger and more expensive gearhead.
Consider current limiting to safely impose limits on gearbox size. With servomotors, output torque is a linear function of current. So besides safeguarding the gearbox, current limiting also 
defends the electric motor and drive by clipping peak torque, which can be from 2.5 to 3.5 times continuous torque.
In each planetary stage, five gears are at the same time in mesh. Although it’s impossible to totally eliminate noise from this assembly, there are many ways to reduce it.
As an ancillary benefit, the geometry of planetaries matches the form of electric motors. Thus the gearhead could be close in diameter to the servomotor, with the result shaft in-line.
Highly rigid (servo grade) gearheads are usually more expensive than lighter duty types. However, for fast acceleration and deceleration, a servo-grade gearhead could be the only sensible 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 effects of free shaft motion.
Servo-grade gearheads incorporate several construction features to minimize 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 costliest of planetaries.
The type of bearings low backlash planetary gearbox supporting the output shaft depends on the load. High radial or axial loads generally necessitate rolling component bearings. Small planetaries can often get by with low-price sleeve bearings or other economical types with fairly low axial and radial load capability. For larger and servo-grade gearheads, durable result shaft bearings are often required.
Like most gears, planetaries make noise. And the faster they operate, the louder they get.
Low-backlash planetary gears are also obtainable in lower ratios. Although some types of gears are generally limited to about 50:1 and up, planetary gearheads expand from 3:1 (solitary stage) to 175:1 or more, depending on the number of stages.