"In the next step, some balls will be removed. The
reason this is done is to prevent headset failure from
brinelling. By leaving the balls room to move around
relative each other it guarantees that any brinelling
that occurs is in random locations. When ball retainers
are used or the cup is left full, the brinelling occurs
in the same places over and over again until it reaches
a noticeable depth and causes headset failure."

Barnett's Manual 5th Edition