Precisely for the reason that is shown in your screenshot. Several pins have been tied off to various power rails, but if you connect them directly in the layout or the schematic, then they get shorted, and one of the pins disappears. If you cut the connection with a metal resistor (which is just a normal piece of metal, just identified as a resistor), then the two nets don't get merged. The power supply remains on one side and the pin on the other.