What is the purpose of the resistor \$R_L\$ in active peak detector circuit?
Is there any use of \$R_L\$ ?


If the circuit doesn't contain \$R_L\$, there is no path to discharge C. The op-amp can't discharge C because there is a diode in the way - the op-amp has only the ability to charge C up to a larger voltage.

So, \$R_L\$ is there to slowly discharge C i.e. it's a slow reset circuit. If there wasn't an \$R_L\$ component you would see the red waveform below for Vout i.e. it is a true peak detector with no reset: -



With \$R_L\$ present you get a slow discharge like this and you are able to detect the final peak (at the expense of a slowly deteriorating "hold" value: -



Picture from here.

So, it's a peak detector that responds quickly and "saves" the largest voltage it's just been subject to and slowly deteriorates that saved voltage so that the next peak can be detected. An alternative to the resistor is a logic control line and a discharge MOSFET: -



Just like in one of your previous questions about the integrator, you can use a resistor and get an approximately good performance without the need for reset intervention OR, you have a reset mechanism that isn't automatic.