Connection between an IA input and a preamp's DC/DC converter
The INA117 is a High Common-Mode Voltage Difference Amplifier, so, this circuit would work well if the CMMR was infinite and the input voltage offset was zero:

It would also work with decent precision with the CMMR and input voltage offset in the order of mV, if instead of a 50 mV shunt, a 10 V shunt was possible (but this would result in significant reduction of the load current).
So, to amplify the shunt voltage, a precision op. amp. is used, but this component can't handle the high common mode voltage. So, the power rails of this op. amp. must &quot;follow&quot; the high input voltage, by being tied to it using a low resistance path.

By making the connection you mentioned, all the circuit parts surrounded by the squares will follow the high common mode input voltage. This is possible since the OPA power supply is isolated from its power input, so its potential can be &quot;moved&quot; to the desired voltage level (like the secondary of a transformer).
This image, from the dc-dc converter datasheet shows the internal simplified block diagram of the two isolated circuits:

The COM pin on the right can be tied to any voltage within the component isolation specs and the \$+V_{OUT}\$ and \$-V_{OUT}\$ will provide voltages relative to this reference.