When the input signal con_pwn is at the high level (Ui), the diodes D 1 and D2 are turned on, and the transistor Q 1 is turned off. The voltage applied to the input of 4427 is UI-2 * 0.7V；;

That is, the voltage of the capacitor C 1, when the input signal con_pwn is low, the diodes D 1 and D2 are turned off, the transistor Q 1 is turned on, and the capacitor is discharged through R 1 and Q 1;

Why this circuit is needed probably depends on the circuit performance of the input signal con_pwn.