If you don't understand that person intimately

You don't need to understand another person to help them. Even if you do understand another person well enough to know what triggered them, telling them can be invasive and therefore needs some amount of implicit of explicit permission.

Being there and being a stable anchor is often better than trying to interfere with their state. That means if you are mentally flexible about changing your state opening up on your side and allowing the emotions to rise in you to a level that similar to the other person but more calm. If you are not flexible and can meditate, that usually a good state to go to.

For me the only reason to leave is if I'm myself not in a stable emotional place. But I can certainly understand if other people generally don't see themselves in a position to help.