An interesting point from fantasy writer Sir Terry Pratchett:
“You can’t build a plot out of jokes. You need tragic relief. And you need to let people know that when a lot of frightened people are running around with edged weaponry, there are deaths. I’m not writing ‘The A Team’–if there’s a fight going on, people will get hurt. Not letting this happen would be a betrayal.”
–Sir Terry Pratchett
And that, my friends, is as good an explanation as any of why good authors write profane scenes. Something has to happen, and it has to be honest.