Netflix is in the video distribution business.  Their usage is hugely variable over the course of a day.  AWS is in the cloud computing business.  They are good at selling unused server time.  If netflix ran their own datacenters, they would have to pay for infrastructure to sit unused for most of the day to meet their peak demand.  With AWS someone else is using and paying for that infrastructure in off hours.  The cost-savings for netflix and synergy for AWS make the deal make sense.