So sub events are ONLY for sprite animations?
No, sub-events can be for pretty much anything, they just happen after the main event has been checked and is true. They are often used where there is something in common that you could check first.
For example, you might need to check lot's of different things, but only when an object is on screen. Instead of lot's of separate events that each check Is On Screen as one of their conditions, you could do Is On Screen as the main event, and then make all the other events sub-events of that.