I think they're used for assorted things, as the name implies they "trigger" things in sessions. What I've used them for is to prevent two trains from entering the same stretch of single track at one time from opposite ends. If both trains happen to hit the signals guarding the single track block at the same instant, both would get a yellow and both would collide in the middle. Set a trigger just after the signal at one end and just before the signal at the other end, then add the "trigger multiple signals" rule to the session, with each trigger causing the signal at the opposite end to go red when a train trips the trigger. So by themselves the triggers don't do anything at all, they have to be set up to do something in the session rules.
