Perhaps the most apparent is to improve precision, which really is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the center distance of the tooth mesh. Sound can be affected by gear and housing materials in addition to lubricants. In general, expect to spend 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 manage the motor’s result torque. What’s more, if you’re using a multi-stage gearhead, the result stage must be strong enough to absorb the developed torque. Obviously, using a more powerful motor than required will require a bigger and more costly gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, output torque is definitely a linear function of current. Therefore besides safeguarding the gearbox, current limiting also shields 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 get rid of noise from such an assembly, there are many ways to reduce it.
As an ancillary benefit, the geometry of planetaries matches the form 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 usually more expensive than lighter duty types. However, for quick 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 resulting from the spring action increases backlash, low backlash planetary gearbox compounding the effects of free shaft movement.
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-equipment shafts. Stiff or “rigid” gearheads tend to be the most costly of planetaries.
The type of bearings supporting the output shaft depends upon the strain. High radial or axial loads usually necessitate rolling element bearings. Small planetaries could get by with low-price sleeve bearings or other economical types with relatively low axial and radial load ability. For bigger and servo-grade gearheads, durable output shaft bearings are usually required.
Like most gears, planetaries make sound. And the faster they run, the louder they obtain.
Low-backlash planetary gears are also available in lower ratios. While some types of gears are generally limited by about 50:1 or more, planetary gearheads extend from 3:1 (single stage) to 175:1 or even more, depending on the number of stages.
