Many writers use tools to check and see if their content has been plagiarized. I also use copyscape to be sure that what I write for clients is completely original and has not been worded similarly by someone else.
Moreover, when you are are writing articles from research, it is easy to sometimes mistakenly quote or paraphrase something you have researched. It may be just a phrase that has stuck in your mind, but you may want to be sure that you do't use it unless you have the author's written permission to use it.
I have also had clients ask me to use certain taglines and after looking them up, found them repeated on several competitors' websites. People who don't write for a living often don't understand what plagiarism is and the threat to their business it holds.
So there are many reasons to use duplicate content checkers, besides intentional duplication of content.