Without it, you can't customize jumping (doing it with events) and still have jump sustain.
Explanation for those who don't get it:
When you put the action:
Z is pressed, Is on Ground (or something like that so you don't float into space)
+Jump
You can't use Jump Sustain. So if you want to set up some fancy double jumping (which needs events with Jump Action) you can't have jump sustain. When you hold down Jump Key normall, it will use jump sustain, but never if the jump is triggered with events.