Perhaps the most apparent is to improve 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 materials and also lubricants. In general, be prepared to pay more for quieter, smoother gears.
Don’t make the error of over-specifying 
the engine. Remember, the input pinion on the planetary should be able handle the motor’s output torque. Also, if you’re using a multi-stage gearhead, the result stage must be strong enough to absorb the developed torque. Certainly, using a better motor than necessary will require a bigger and more expensive gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, result torque is usually a linear function of current. So besides protecting the gearbox, current limiting also shields the engine and drive by clipping peak torque, which can be anywhere 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 get rid of noise from such an assembly, there are many methods to reduce it.
As an ancillary benefit, the geometry of planetaries fits the shape 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 speedy acceleration and deceleration, a servo-grade gearhead may be the only wise choice. In this kind of applications, the gearhead could be seen as a mechanical springtime. The torsional deflection caused by the spring action adds to backlash, compounding the consequences of free shaft motion.
Servo-grade gearheads incorporate many construction features to minimize torsional stress and deflection. Among the more common are large diameter output shafts and beefed up support for satellite-equipment shafts. Stiff or “rigid” gearheads tend to be the costliest of planetaries.
The type of bearings supporting the output shaft depends upon the strain. High radial or axial loads generally necessitate rolling element bearings. Small planetaries can often manage with low-price sleeve bearings or various other economical types with relatively low axial and radial load ability. For bigger and servo-grade gearheads, heavy duty result shaft bearings are often required.
Like most gears, planetaries make sound. And the faster they run, the louder they get.
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