Why A short HF mobile whip is electrically very short, so the current node at the tip means the RF voltage there is extremely high, often thousands of volts. A sharp point concentrates that electric field and lets it leak away as corona discharge, wasting power and eroding the tip. A corona ball is a smooth metal sphere that spreads the field over a larger radius, raising the voltage needed to start a discharge. As a bonus it adds a little top capacitance, but its job is suppressing the discharge.
Watch out The choice about striking objects sounds plausible because a ball on the end looks like a safety tip, but corona balls are about electric field control, not mechanical protection. The Q and bandwidth choices are backwards in emphasis: the added top capacitance slightly helps loading efficiency, it is not the reason the ball is there.
Corona = sharp points leak RF. Round the tip, blunt the field, keep the watts in the antenna.