Some vehicles exhibit extreme lag in their actuators, for example quads with oversized, heavy propellers.
This means Stabilization inner loop might be very slow to arrest or control rotation rate.
Although this is still controllable in rate mode, it means the outer loop for attitude mode would have to be tuned extremely weakly to not induce oscillations, which turns attitude mode almost useless.
A feed forward term that doesn't just look at the current attitude but also the current rotation rate (and assumes that no matter what the setpoint is set to, this rotation rate will be ongoing for some time into the future) might behave significantly better and achieve better and more prompt control results for the same coefficients, when in attitude mode.
by default this feature would be disabled (feed forward term set to 0) but expert users might want to enable it if they need good responses from extraordinary large or sluggish vehicles.