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 is also affected by gear and housing materials and also 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 handle the motor’s output torque. What’s more, if you’re using a multi-stage gearhead, the result stage must be strong enough to soak up the developed torque. Certainly, using a better motor than necessary will require a bigger and more expensive gearhead.
Consider low backlash planetary gearbox current limiting to safely impose limits on gearbox size. With servomotors, result torque is certainly a linear function of current. So besides protecting the gearbox, current limiting also protects 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 this assembly, there are many methods to reduce it.
As an ancillary benefit, the geometry of planetaries fits the form of electric motors. Therefore 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 speedy acceleration and deceleration, a servo-grade gearhead may be the only sensible choice. In such applications, the gearhead may 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 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 most costly of planetaries.
The kind of bearings supporting the output shaft depends upon the strain. High radial or axial loads usually necessitate rolling element bearings. Small planetaries can often get by with low-cost sleeve bearings or various other economical types with relatively low axial and radial load capability. For larger and servo-grade gearheads, durable result shaft bearings are usually required.
Like most gears, planetaries make noise. And the quicker they run, the louder they get.
Low-backlash planetary gears are also obtainable in lower ratios. While some types of gears are generally limited to about 50:1 and up, planetary gearheads lengthen from 3:1 (single stage) to 175:1 or even more, depending on the number of stages.