Why do some switching regulator ICs have single VCC pin but multiple PGND pins?
A regulator chip senses output voltage,
its difference from an internal reference voltage determines
the correction requirement.   So, if a power-carrying ground
pin has even just a few millivolts of resistive voltage drop
between the circuit board and the silicon chip, THAT could
cause a regulation error by adding to the voltage reference.

To combat that, a second ground pin could be used, for the voltage reference, so that there is a low-current wire bonding that
second ground pin for improved accuracy. 
Less often, the ground voltage from a second sense wire that
attaches to the distant load is the 'common' connection for such a reference voltage (but wouldn't be labeled as 'gnd').
A/D converters, too, may have both a 'digital ground' and 'analog
ground'.

Sometimes a pin can be used at the factory for an adjustment, or test,
but has no useful function in normal chip operation; the grounding of such an unused (but possibly sensitive) circuit node can be strongly 
recommended.   Labeling such a pin 'gnd' ensures compliance.