This circuit is suitable for measuring floating voltage. Specifically, the case is a solar panel with disconnectable negative pole. E.g. during the night you want to disconnect it to prevent current flowing from the battery (on the right side) to the panel (on the left side). The diode on the left side rapresent the body diode of the mosfet which disconnect the panel's ground. The diode on the right side rapresent the body diode of a mosfet in the buck stage (MPPT).
By measuring the voltage on the PNP collector, one can know the current flowing in the collector and the base (Ib=Ic/Hfe), so can calculate the volage between base and V+ (emitter resistor is known because fixed by design) with Veb=0.6+Ie*Re, thus the panel's voltage by multipling it by 2 (which is the voltage divider in the base).
