The gain of a charge-sensitive amplifier (CSA) is Vo/Qi~1/Cf, where Vo is the output voltage, Qi is input charge, and Cf is the feedback capacitor.
Why can't I get rid of the whole op-amp part of the circuit and directly use a voltmeter to measure the voltage drop, V, on Cf?
That V is calculated as Qi/Cf, exactly the same as Vo.
The internal resistance Rf of the voltmeter can be large so that most charges will be collected by Cf. The slow discharge of Cf over Rf will bring the voltage back to zero gradually so that the circuit is ready for the next measurement.
My specific application of this circuit is for radiation detection. The radiation detector can be seen as a combination of a capacitor and a current source if it is hit by radiation. We normally use a feedback resistor to bring back the baseline gradually.