Because the load is divided between several planet gears instead of being carried by one pair of meshing teeth. Three or more contact points share the torque, so each tooth carries a fraction of the total and the whole train can be much smaller for a given output. That load sharing is the single reason planetary arrangements dominate wherever torque density matters.

The output shaft is on the same centreline as the input, so a planetary gearmotor is a straight assembly with no offset. That makes it easy to mount in line with a driven shaft, simple to integrate into a machine frame, and compact in cross-section. It also means the unit cannot be used to solve a layout problem needing an offset or a right angle.

Because they combine the three properties a positioning axis needs: high stiffness, low backlash and low inertia. Load sharing between planets provides the stiffness, precision manufacture provides the low backlash, and the compact rotating masses keep added inertia small. Reduction also reduces reflected load inertia by the square of the ratio, which is frequently the actual reason a gearhead is fitted.

Standard industrial planetary units carry moderate backlash suitable for general drives, while precision and low-backlash grades are specified in arcminutes, with the tightest grades reaching very low single figures. Cost rises sharply as backlash falls, so specify what the application genuinely requires: on a conveyor or a pump the figure is irrelevant, and paying for a precision grade is wasted.

By stacking stages in line, since each planetary stage is short and the output remains coaxial. Two and three stage units are common and add relatively little length for a large increase in ratio. Efficiency falls slightly with each stage as with any gear train, so where an extremely high ratio is needed it is worth comparing a multi-stage planetary against alternative arrangements.

Yes, comparable with helical gearing at high nineties per cent per stage, because the teeth transmit torque by rolling contact rather than sliding. That makes them suitable for continuous duty as well as for positioning, and it means they generate relatively little heat, which matters in the compact housings the format is chosen for in the first place.

Robotics and automation, positioning and indexing axes, machine tools, packaging machinery, wind and solar tracking drives, mobile and off-highway equipment, and any drive where the torque required will not fit in the space available for a conventional gearmotor. The deciding factor is usually a combination of torque density and either stiffness or the need for a coaxial output.