Pin 7 discharges the
capacitor after it has fully charged. So, if you made the resistor almost zero, the capacitor would discharge very quickly.
This produces an output that is mostly high but with a very short low time.
If you could make the resistance zero, the output would stay high all the time but there is a risk that the transistor that discharges the capacitor (inside the chip) could get damaged, especially if a large electrolytic capacitor was used.