In shunt with the feedback upper resistor, it gives you some "D"
for your PID, improving load-step response by "advance notice"
(and defeating somewhat, the input and routing capacitance of
the center, feedback node which causes feedback lag).
In shunt with the "lower" resistor a cap would add phase lag and
destabilize your loop (or increase the compensation needed, which
will hurt transient response).
Check out "Type 1", "Type 2", "Type 3" DC-DC compensation.