Perhaps the most apparent is to improve 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 along with lubricants. In general, be prepared to spend more for quieter, smoother gears.
Don’t make the error of over-specifying the electric motor. Remember, the input pinion on the planetary must be able deal with the motor’s output torque. What’s more, if you’re utilizing a multi-stage gearhead, the output stage must be strong enough to soak up the developed torque. Obviously, using a better motor than necessary will require a larger and more expensive gearhead.
Consider current limiting to safely impose limits on gearbox size. With servomotors, result torque is definitely a linear function of current. Therefore besides safeguarding the gearbox, current limiting also protects the electric motor 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 this assembly, there are many ways to low backlash planetary gearbox reduce it.
As an ancillary benefit, the geometry of planetaries fits 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 generally more costly than lighter duty types. However, for quick acceleration and deceleration, a servo-grade gearhead may be the only 
wise choice. In such applications, the gearhead could be viewed as a mechanical spring. The torsional deflection resulting from the spring action adds to backlash, compounding the effects of free shaft movement.
Servo-grade gearheads incorporate many 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 most costly of planetaries.
The type of bearings supporting the output shaft depends on the strain. High radial or axial loads usually necessitate rolling component bearings. Small planetaries could get by with low-cost sleeve bearings or additional economical types with relatively low axial and radial load capability. For bigger and servo-grade gearheads, durable output shaft bearings are usually required.
Like most 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 generally limited by about 50:1 and up, planetary gearheads extend from 3:1 (one stage) to 175:1 or more, depending on the number of stages.
