Everything scripted within the "hidden_tooltip" scope will be triggered normally by the game, but not shown in the tooltip.
This can be necessary for some intrigue events, for example assassination events. They trigger another events for the attacker, but if the target could see this effect ("<character name> receives an event"), he would know who sent the assassin.
Another thing are long event chains where several events are triggered in a row, but the player doesn't need to know about that (because it are just flavor events), so the additional tooltip info could be confusing.
There is also an opposite command, called "tooltip":
Everything that is added in there will be shown in the tooltip, but not actually executed. This is useful if the actual effects of an eventchain are applied by an event from another character. For example, your character triggers the event that boosts relations with a courtier, but the opinion modifer from this event chain is not it is applied by an event from the courtier. Normally, the player could not see the final result, but with "tooltip", you can add a fake tooltip entry that shows the exact outcome.