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 is also affected by gear and housing components along with lubricants. In general, expect to spend more for quieter, smoother gears.
Don’t make the mistake of over-specifying the motor. Remember, the insight pinion on the planetary should be able handle the motor’s result torque. Also, if you’re utilizing a multi-stage gearhead, the result stage must be strong enough to soak up the developed torque. Obviously, using a better motor than necessary will require a bigger and more costly gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, result torque is certainly a linear function of current. Therefore besides safeguarding 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 simultaneously in mesh. Although it’s impossible to totally remove noise from such an assembly, there are several methods to reduce it.
As an ancillary benefit, the geometry of planetaries matches the form of electric motors. Hence the gearhead can 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 rapid acceleration and deceleration, a servo-grade gearhead may be the only sensible choice. In such applications, the gearhead may be seen as a mechanical springtime. 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 minimize torsional stress and deflection. Among the more common are large diameter output low backlash planetary gearbox shafts and beefed up 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 load. High radial or axial loads generally necessitate rolling element bearings. Small planetaries can often get by with low-price sleeve bearings or other economical types with relatively low axial and radial load capability. For bigger and servo-grade gearheads, heavy duty result shaft bearings are often required.
Like most gears, planetaries make sound. And the faster they operate, the louder they get.
Low-backlash planetary gears are also available in lower ratios. Although some types of gears are usually limited to about 50:1 or more, planetary gearheads prolong from 3:1 (solitary stage) to 175:1 or even more, 
depending on the number of stages.