I have had 2 births now when uterine "massage" was used.
The first (birth 2) was a retained placenta and PPH when the placenta had to be manually removed under GA but before they put me under there was quite a lot of trying to get it out.
The second (birth 4) I had a secondary PPH about 2 hours post birth. The massage was done (along with synto drip) to expel blood clots and help contract my uterus down which had filled with blood.

The other births there was no problems so it wasn't used at all.

From my understanding of it, massage is used to help the uterus contract when it's not contracting on it's own as the contracting down after birth is what seals off the blood vessels from where the placenta was attached.