That part makes a negative voltage rail that allows the LT3088 output adjustment to go all the way down to 0V. 
The LT3088 has a minimum load of around 2mA, so the LT3088’s output will never go to 0V unless there is a pull-down to a negative voltage.…

Hi Santiago,

    The design that you are considering is not trivial, and there
are several factors that you will have to consider. The voltage
reference is key, of course, but you also need to have a low-offset,
low-drift amplifier for your gain loop, and…