Netflix and YouTube know through Big Data just what a person has viewed and his/her behaviour online. Based on this information, the person will be shown different recommendations. For example, if a person has viewed a couple of horror films from start to end, Netflix will know that the viewer is interested in horror films.
Accordingly, Netflix will recommend a few more horror films. The viewer is pleased with this easy access to his/her favourite genre, and continues to be a customer, thus increasing Netflix revenue.