Way back in 2013/14, Cloud Access Security Brokers (CASBs) were first deployed to identify Shadow IT, or unsanctioned cloud applications. At the time, the prevailing mindset amongst security professionals was that cloud was bad, and discovering Shadow IT was viewed as the first step towards stopping the spread of cloud in their organization.
Flash forward just a few short years and the vast majority of enterprises have done a complete 180º with regards to cloud, embracing an ever increasing number of "sanctioned" cloud apps. As a result, the majority of CASB deployments today are focused on real-time data protection for sanctioned applications - typically starting with System of Record applications that handle wide swaths of critical data (think Office 365, Salesforce, etc). Shadow IT discovery, while still important, is almost never the main driver in the CASB decision making process.