Simple Circuit Analysis, why this cap is needed?
Whenever the Q output goes high the transistor will briefly and weakly turn on, provided it's been off for long enough for C1 to discharge. Q1 will only be able to conduct 20mA or so typically. The time constant is a couple hundred milliseconds. 

R1 does nothing of value, most likely (it might have some small effect if the circuit is triggered immediately after a brief power blip) 

The voltage divider R2/R3 ensures that the transistor does not see -15V when C1 is discharging, which could damage the transistor (it only sees a few volts negative), and it also reduces leakage current through Q1 when off by diverting Icbo to ground.  

Here's a plot of what it does with a 10K load: 



Red trace is Q output of FF, green trace is voltage across load (not to ground).