First you can write articles that inform people of something. This can be based on a product, a review of a product or service, or just general tips and tricks about anything.
Often, established sites need content and the owner doesn't have the time or skills to write it. This is why they hire ghostwriters.
Others build entire sites of ghostwritten content to make money on monetization (adsense, amazon, etc...).
This is how many great sites are built but a lot of attention has to be put into great content, high quality potent copy that people love. It's not as easy as it seems actually but it works and it means there are a lot of opportunities to write out there.
Often established site owners want a continuous “drip” of content added to the site and will hire you to write that content. They may have become excited about the content when they built the site and their interest in the topic wanes.
This is when they, to keep their site alive and earning them money, ranking, and overall being an authority site, hire writers to keep the content flowing. Sites that continue to add content continue to look fresh on Google if it's written tightly. Some people just hire ghostwriters to rewrite THEIR content to update it, keep it fresh to search engines and valid in a post panda world.
Others want to clone their highly performing money making sites into new angles of the same niche but need fresh new content for it. The reasons they could need content are mind boggling, somewhere in the millions of articles needed every day range...
They also come for content to replace robot created content (i.e. spun content or content scraped with a plugin from other sites i.e. not original) they purchased on a site from flippa or other website auction sites. Some people buy a site just for the structure then hire ghostwriters to fill in the actual content.
This could be a lot of content and a great writer means a site full of work that's consistently written. Think of it in terms of how much they will pay you to turn their site's lights on in the darkness of the web! Content is king and for writers it's potential money. The better a writer you become in this career, the more you can demand for your services- and people will pay it if you're consistent, write quality, and your content earns them traffic (which translates into potential money).