Perhaps the most apparent is to increase precision, which is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the center distance of the tooth mesh. Sound is also suffering from gear and housing components in addition to lubricants. In general, expect to pay out more for quieter, smoother gears.
Don’t make the error of over-specifying the motor. Remember, the input pinion on the planetary must be able deal with the motor’s output torque. Also, if you’re utilizing a multi-stage gearhead, the output stage should be strong enough to soak up the developed torque. Obviously, using a better motor than necessary will require a larger and more costly gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, output torque is certainly a linear 
function of current. Therefore besides protecting the gearbox, current limiting also shields the engine 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 concurrently in mesh. Although you can’t really totally remove noise from this assembly, there are many ways to reduce it.
As an ancillary benefit, the geometry of planetaries fits the shape 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 costly than low backlash gearbox lighter duty types. However, for rapid acceleration and deceleration, a servo-grade gearhead could be the only wise choice. In such applications, the gearhead may be viewed as a mechanical springtime. The torsional deflection caused by the spring action increases backlash, compounding the consequences of free shaft motion.
Servo-grade gearheads incorporate a number of construction features to minimize torsional stress and deflection. Among the more prevalent 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 kind of bearings supporting the output shaft depends upon the strain. High radial or axial loads usually necessitate rolling component bearings. Small planetaries can often manage with low-price sleeve bearings or various other economical types with relatively low axial and radial load capacity. For bigger and servo-grade gearheads, durable output shaft bearings are often required.
Like the majority of gears, planetaries make sound. And the faster they operate, the louder they get.
Low-backlash planetary gears are also obtainable in lower ratios. While some types of gears are usually limited by about 50:1 and up, planetary gearheads prolong from 3:1 (single stage) to 175:1 or even more, depending on the amount of stages.