if nothing else it uses precious radio time
Far better a single extra word and clarity than a phrase susceptible to misinterpretation, an incorrect read back or worse, a conflict.
if they say reduce to two one zero, does that mean descend to FL 210??? reduce heading to 210 degres? where exactly is the confusion
It's intended for phrases that contain multiple instructions but for the sake of consistency it is used all the time.
e.g. "Dimbo123, descend flight level 120, turn left heading 110 degrees, speed 220 knots when level".
Which is far clearer than:
"Dimbo123, descend 120, head 110, 220 when level".
Or do you disagree?