Perhaps the most apparent is to increase precision, which really is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the guts distance of the tooth mesh. Sound can be suffering from gear and housing materials along with lubricants. In general, expect to pay more for quieter, smoother gears.
Don’t make the error of over-specifying the electric motor. Remember, the input pinion on the planetary should be able deal with the motor’s output torque. What’s more, if you’re utilizing 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 limitations on gearbox size. With servomotors, output torque can be a linear function of current. So besides safeguarding the gearbox, current limiting also defends the electric motor 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 eliminate noise from such an assembly, there are several methods 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 usually more expensive than lighter duty types. However, for rapid acceleration and deceleration, a servo-grade gearhead may be the only wise choice. In this kind of applications, the gearhead may be seen as a mechanical spring. The torsional deflection caused by the spring action increases backlash, compounding the effects of free shaft motion.
Servo-grade gearheads incorporate a number of construction features to reduce torsional stress and deflection. Among the more prevalent are large diameter output shafts and beefed up support for satellite-gear shafts. Stiff or “rigid” gearheads tend to be the costliest of planetaries.
The type of bearings supporting the output shaft depends on the load. High radial or axial loads low backlash planetary gearbox generally necessitate rolling component bearings. Small planetaries can often manage with low-cost sleeve bearings or other economical types with fairly low axial and radial load ability. For larger and servo-grade gearheads, durable output shaft bearings are usually 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 extend from 3:1 (one stage) to 175:1 or even more, depending on the number of stages.