It is used to turn the normally constant-output-voltage chip into a constant current chip.
Q2 to Q5 convert the voltage across Rcs (referenced to the positive rail) into a voltage across R3 referenced to the 0V rail.
I had a search around to make sure I knew how it worked, and a Zetex (now Diodes) app note and data sheet cropped up describing ZDS1009, a monolithic equivalent to Q2 – Q5 (left – I say monolithic, but it might not be, as Zetex was a dab hand at ‘system-in-package’ before that was a thing).
Looks like this one works from a handful of volts up to 30V, and can take a bit of a pounding as, despite normally working at milliamps, maximum current is 1A.
Max output offset voltage is 4mV across R4, although apparently 70% of chips have an offset below 300μV.
The app note describes operation in detail.
To highlight a extremely clever current sense circuit, that can be powered from 2.7 – 18V while sensing current through a resisistor at −16 to +80V, take a look at Texas Instrument’s INA19x family (INA193 – INA198) (left).