Perhaps the most apparent is to increase 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 as well as lubricants. In general, be prepared to pay more for quieter, smoother gears.
Don’t make the mistake of over-specifying the engine. Remember, the input pinion on the planetary should be able deal with the motor’s result torque. Also, if you’re using a multi-stage gearhead, the output stage should be strong enough to soak up the developed torque. Obviously, using a better motor than required will require a bigger and more expensive gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, result torque is usually a linear function of current. Therefore besides protecting the gearbox, current limiting also protects the electric motor and drive by clipping peak torque, which may 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 remove noise from this assembly, there are several ways to reduce it.
As an ancillary benefit, the geometry of planetaries matches the shape of electric motors. Thus the gearhead could 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 low backlash planetary gearbox Servo-grade gearhead could be the only sensible choice. In such applications, the gearhead may be seen as a mechanical spring. The torsional deflection resulting from the spring action adds to backlash, compounding the consequences of free shaft motion.
Servo-grade gearheads incorporate many construction features to minimize torsional stress and deflection. Among the more prevalent are large diameter result 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 can often manage with low-cost sleeve 
bearings or additional economical types with fairly low axial and radial load capability. For larger and servo-grade gearheads, durable output shaft bearings are often required.
Like the majority of gears, planetaries make sound. And the quicker they run, the louder they get.
Low-backlash planetary gears are also available in lower ratios. While some types of gears are usually limited by about 50:1 or more, planetary gearheads prolong from 3:1 (single stage) to 175:1 or more, depending on the amount of stages.