Here's another reason people may write for free is to establish their 'expert' status. Most of the time these are small business owners or staff of small or home businesses. It is much easier to find these bloggers in more common topics like (but not limited to) parenting, health. Many of these businesses will jump at the opportunity to 'guest blog' or be a resident blogger in a special topic in exchange for exposure. Of course, if you're asking the business owners crowd, they'll usually want to know stuff like your traffic and readership. They like numbers to justify their time investment. Good luck!