Perhaps the most apparent is to improve precision, which is a function of manufacturing and assembly tolerances, gear tooth surface finish, and the guts distance of the tooth mesh. Sound can be affected by gear and housing components and also lubricants. In general, be prepared to pay more for quieter, smoother gears.
Don’t make the error of over-specifying the motor. Remember, the input pinion on the planetary must be able manage the motor’s output torque. Also, if you’re using a multi-stage gearhead, the result stage should be strong enough to soak up the developed torque. Certainly, using a more powerful motor than required will require a bigger and more expensive gearhead.
Consider current limiting to safely impose limitations on gearbox size. With servomotors, output torque is usually a linear function of current. Therefore besides safeguarding the gearbox, current limiting also shields the engine 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 concurrently in mesh. Although it’s impossible to totally get rid of noise from this assembly, there are many methods to reduce it.
As an ancillary benefit, the geometry of planetaries matches the form 
of electric motors. Therefore 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 speedy acceleration and deceleration, a servo-grade gearhead could be the only sensible choice. In such applications, the gearhead could be viewed as a mechanical springtime. The torsional deflection caused by the spring action adds to backlash, compounding the consequences of free shaft motion.
Servo-grade gearheads incorporate several construction features to reduce torsional stress and deflection. Among the more common are large diameter result shafts and beefed up support for satellite-equipment shafts. Stiff or “rigid” gearheads tend to be the costliest of planetaries.
The type of bearings supporting the output shaft depends upon the load. High radial or axial loads usually necessitate rolling component bearings. Small planetaries can often manage with low-cost sleeve bearings or additional economical types with fairly low axial and radial load capacity. For larger and servo-grade gearheads, durable output shaft bearings are often required.
Like most gears, planetaries make sound. And the faster they operate, the louder they get.
Figure out every little thing you’ve ever wished to know about low backlash gearbox.