Great content designers are almost invisible. They ensure that you get the information that you need at the right time and in the right place. But because you are so immersed in successfully completing your task, you aren't even aware of the writing. (In software, you only really notice when things go wrong and the information is hard to find or understand.)
I don't just write. I plan out your experience. I tweak how you interact with the interface. I ask challenging questions so that you have all the answers, without needing to ask.