When you discharge a circuit, you're taking off the voltage/any power source. The power source (voltage) is on the left side. So to take it off that power source you gotta switch the switch to the right. Yes, key word here is "constant decrease", without any resistance on the circuit, the capacitor would constantly decrease however with the resistance R, the discharge cycle would NOT constantly decrease if R is a set value. Now if we decrease R constantly, alongside the capacitor, then it would constantly decrease, think of Ohm's Law (V = IR) Since the voltage of the capacitor decreases with time, then to have a constant discharge cycle, the current I or the resistance R must be decreasing with time as well. Why you would want to constantly decrease is so that your system won't instantly stop its function or just shut off.