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I think they have just used spare pins on the package to ease pcb layout. Not 100% on this. I know they work with either connected, but I still try to use both. Useful if you need to transfer power across the chip to save long track runs.
As elmuhamady sayd; to improve the dynamic current distribution, and for less "ground bouncing", because PICs are builded as CMOS ICs; they switches with relative high-speed & more high currents trough transition times as a "normal 8051".....
This current knock/push makes disturbances/potential swifts in resistances & inductances of GND/Vcc lines_called GND-bounces...
Their are to eliminate-minimum to become smaller_ trough more or parallel used pins.