They are the unnecessary hand disinfectant of the internet.
Trigger warnings, for the blissfully unaware, are notifications preceding amateur writings or slapped on by amateurs ahead of classic works with the intent of warning people about 'jarring content' so they aren't 'triggered'.
On the surface this is a courtesy, and that is how it is sold to people. That to not use these things makes you a jerk at best and a monster at worst. In truth, if you truly wish to do humanity a service you should never ever use them at all.
Trigger warnings are often used by the foolish to filter out all jarring or unpleasant information to preserve a sort of bubble where they never have to deal with disturbing details. The reason this is folly however is that it leaves them emotionally and mentally weaker when the bubble- as bubbles are wont to do- inevitably pops.
You could look at it as being similar to the human immune system. It must be tested in order to remain strong. If you live in a overly sterile environment for too long, you will find yourself sickened more often and more dramatically when you leave it by virtue of having atrophied your body's natural defenses.
The same thing is true of unpleasant media. If you filter it all out, you can easily forget how to deal with it like an adult when you are forced to encounter it eventually. Worse, by reinforcing your fear of the given subject matter with avoidance you are actually making yourself more scared than is rational.
Even if you have traumas and bad memories are 'triggered' by certain media the answer is not avoidance but rather measured exposure and desensitization.
Indulging too much in a desire to hide away from all of the unpleasantness of reality and fiction turns you into the mental equivalent of a bubble boy. This is a bad thing for the individual and society alike.
Finally many people use trigger warnings as a means of foisting responsibility for how they respond to stimuli onto someone else's shoulders, which I find odious at best.