The main reason why Cambridge Analytica did this is that they wanted to know where exactly they would place political ads. The political ads are meant to sway people and are meant to make them choose one presidential candidate over the other. Cambridge Analytica has been doing the consulting for a long time now. It was just a surprise for people that their profiles were also analyzed in the process.
Facebook users started to become alarmed when other agencies start to know more about their browsing history and so much more. When the whistleblower confirmed what people have suspected, people have already started to delete their Facebook accounts. Facebook has since addressed this concern and is working on their security even more.
The reason that Cambridge Analytica was buying user information from Facebook was that as a political consulting firm they wanted to use the information so they knew exactly where to place certain political ads. It has been known that Cambridge Analytica has been working and consulting in political races in the United States for almost five years now. The company would look for people who lived in the political contested areas and then cater a political ad towards them.
Facebook users started to complain all around the world as they wondered how third party advertisers were able to know personal information such as browsing histories. The public spoke up and then eventually former employee Christopher Wylie blew the whistle in an interview with The Guardian. In the end, the reason that Cambridge Analytica purchased Facebook user data was to help with the political consulting work that they were doing for political races in the United States.