BQ25570 Ground Layout
Since this is a switching power supply, large current spikes will flow through the ground power path.  Since our conductors are not perfect, parasitic inductance and resistive losses can cause voltage differences from one ground point to another.  In sensitive circuits, this voltage difference can cause problems.  This is why the manufacturer of the chip has given you the option to separate the grounds to isolate the large current paths to specific pins.  The implementation would be to tie the two grounds together at a single point, through a jumper or zero ohm resistor for instance.

In spite of the words that are contained in the datasheet and the layout guide to separate the grounds, the example design directly connects both VSS pins and the PowerPad to ground.