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 suffering from gear and housing components along with lubricants. In general, expect to pay more for quieter, smoother gears.
Don’t make the mistake of over-specifying the electric motor. Remember, the input pinion on the planetary must be able manage the motor’s result 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 more powerful motor than necessary will require a bigger and more costly gearhead.
Consider current limiting to safely impose limits on gearbox size. With servomotors, result torque can be a linear function of current. So besides safeguarding the gearbox, current limiting also protects the motor 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 eliminate noise from this assembly, there are several ways 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 output shaft in-line.
Highly rigid (servo grade) gearheads are generally more costly than lighter duty types. However, for speedy acceleration and deceleration, a servo-grade gearhead could be the only sensible choice. In this kind of applications, the gearhead could be viewed as a mechanical springtime. The torsional deflection caused by the spring action adds to backlash, 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 costliest of planetaries.
The kind of bearings supporting the output shaft depends upon the load. High radial or axial loads usually 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 capability. For larger 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 get.
Low-backlash planetary gears are also available in lower ratios. Although some types of gears are generally limited to about 50:1 or more, planetary gearheads low backlash gearbox prolong from 3:1 (single stage) to 175:1 or even more, depending on the amount of stages.