If you don't want the effects to appear in the tooltip.
For example, let's say you have an event option that gives 100 gold, but has a 25% chance of making a character greedy, but you don't want the player to know exactly what the chance of getting the trait is. You can hide the chance of gaining the trait inside a hidden_tooltip.
oh thats sounds pretty good, thanks.... and one more lol...... what is the point of is_triggered_only ?
EDIT: ALSO.... whilst im here.... is there a way to ensure that a province flag is removed after a certain time, or is the only way to remove it using another event which clears it?