Some universities count Coursera teaching toward regular teaching obligations. In other words, the university pays the teacher to teach on Coursera. Why do this? To officially be a part of Coursera.
One of my colleagues taught a course on Coursera out of interest and is not doing it again. But his course is now fully-automated and can be offered w/o his active participation (and has been).
An example mentioned by Daphne Koller (one of Coursera founders) on her recent visit to Michigan: a professor from a small college was normally getting twelve students in his/her course, but on Coursera, tens of thousands students signed up, and many completed the course -- great for personal fulfillment.
Keep in mind that some people like to teach and don't worry about money.
For example, at Stanford, you may find local professionals teaching for free -- it's a privilege. Coursera is essentially the same way, but less selective and with the bonus that your course will live forever.
More questions on Academia:
- What are the downsides of being a professor?
- Why does academia have so much politics?
- What is it like to be denied tenure as a professor?