They can write your documents, or edit and polish your rough draft, while you get on with the job. Because producing documents is what writers do, they're fast at it. They home in on the essential information, they write and edit quickly, and they know the production pitfalls to avoid. So you save twice - once because one of your people is not tied up writing, and again because the writer spends less time on the project than one of you would have. Some documents are so mission-critical that if the message is unclear or ambiguous, the organisation as a whole suffers.
Bad manuals can actually cost money, in foul-ups, frustrated staff and clients, and litigation.
Here's an example of an unclear sentence with two possible, and quite different, meanings. There are restrictions on making agreements which include having time limits. Good writers will make sure that you say just what you want to say, and that your audience hears you loud and clear. We do, and we are. When you're close to a subject, and you understand it intimately, it is very hard to write about it for an audience that doesn't have your background knowledge.