what are the advantages of connecting two GND polygons only in one place on PCB?
By having separate polygons the designer assures currents flow where he wants them to flow. This is not a law, in some cases indeed single solid ground plane may be better. Still, for most cases, it's easier for designer's mind to see where analog/digital/high power currents flow, if they all have separate grounds and are only connected in a single point.

Ah, forgot to mention. The importance of such controlled current flow comes from noises and interference that current may cause. For example, digital currents may create low amplitude high frequency noise in analog circuits. High power currents may cause anything, if they flow under digital/analog circuits. And this is because even ground plane is not ideal, it has resistance, inductance and capacity. Not to mention complex phenomena created by holes in the plane.

By the way, one way is to indeed have separate nets in the design and connect with jumper resistor, and the other way is to call all grounds the same, and cut the polygon in the layout. Then the connection point will have much less resistance and will allow to pass higher currents.