The Call Another Action Set action is used to run the actions of another control.
This is commonly used so as not to duplicate the same actions in different controls.
Example: Imagine four buttons that can save a record:
All of these buttons have the same actions except for minor differences. You can put the common save actions in the first button, and then use Call Another Action Set from the other three buttons with additional actions to handle their special cases. This way, if there is any change to the core save process, you will only need to update it in one place.