In low power applications, some load currents are quite low, and require a long time to discharge the OUTPUT voltage to low enough voltage to reset itself.
Some loads have internal OTP, or memory, which need to be reset when powering down for proper operation. Some kinds of loads need the voltage down to less than 0.4V to reset itself.
The active discharge resistor is used to discharge the OUTPUT voltage when shut down.
Please note MAX38640/1/3 have the feature, while MAX38642 has not.