Setting aside the difference between warning and trigger (because that is an entirely different issue), one reason I can think of for wanting to be warned for profanity, as well as other things in the comic, is that some people might be looking to read the submissions in a place where such a thing might not be acceptable. For example, someone who works in a conservative workplace might not want their co-workers to see a Karkat fanfic with the word 'fuck' iterated about twenty times, or someone in a library might wish for nudity to be clarified in the OOC comments so that they don't accidentally get themselves kicked out for indecency. People might have read the comic when they weren't somewhere like this, but for some reason or another may not get enough time during the day to read the submissions in a private place.