Ghostwriters do a lot of different things, from writing speeches, prose, and copy "whole cloth," to doing "punch-up" work making sure key parts of a piece land, to working more like a co-writer, consultant, or editor in a collaborative process.
Or, at any rate, those are all things I've done as a ghostwriter. I suppose I can't speak for the whole profession.
Long story short, if you need something written, need someone to write something with, or need to add things to your writing that are outside your range, hire a ghostwriter. Specifically, hire me.
I tend to have more questions than answers in life, which means my personal fiction work and essay work tends to be searching and open-ended. That's great for me, but it means the work takes a long time and isn't easily marketable.
Ghostwriting clients have more specific needs and more organized projects. They also know a lot about their own fields and experiences. That means I get to focus on one particular thing, apply my knowledge of experience, and help someone achieve a concrete goal. There's something very satisfying about solving a concrete problem. That's part of what I always liked in my day jobs, too.
If you're interested in a potential collaboration, please see my contact page.
I find it pretty boring. We don't need larger quantities of writing, and we don't need to smooth off the edges. If you're going to say things, make sure they really mean something. Work with a human.