circuit ground vs chassis ground
Assumptions: It seems that the output of the configuration you have drawn there is a 24V DC signal that is +12 on and -12 with respect to circuit ground. There is a circuit ground from what appears to be the neutral of the ADC and a chassis ground from what appears to be a filtering RC circuit. I believe the intent of the filtering circuit is thus to send high frequency ripples from the ADC to ground.

Answer: Usually when you separate the ground versus the chassis wiring its due to fault currents. The power circuit here is circuit that is solidly grounded. If a fault occurred, the hope is that the fault current would flow to ground via that ground connection. If you connected the power circuit ground to the chassis you could run the risk of the full ADC short circuit running through the body into an unsuspecting homosapian in contact with it. The chassis filtering also provides a higher resistance to ground which might be more desired for the filtering that that particular designer wanted to have.

Edit: I'd like note that grounding a system can be very specific to different applications. This is because you have to consider what the circuit components need as well as what they will be interacting with. In this example the chassis grounding could be an attempt to reduce circuit noise. However, other systems use chassis grounding because there is no effective grounding pin between the interconnecting devices. These must be doubly insulated to prevent the chassis from getting energized where a user may be able to contact it. An example of this is when you see a 3 pin wire vs a 2 pin wire in common North American electrical devices. 

Hope that helped clear up some confusion.