I think it might be to help you find and use your big people words instead of the easy, default response. Swearing is just an automatic habit and therapy is about learning to be able to choose. Have you ever noticed how, when a therapist swears, it can sound a little forced? That's because they have chosen to swear, the swearing has not chosen them. The swearing has no effect if it's automatic, it's meaningless and devalues you and what you have to say.