We started with a very clear primary use case, which is what landed us on Bitglass: The need to protect Office 365. More specifically, we wanted to make sure that untrusted devices would not be able to download the full O365 client.
Granted, you can do that with Microsoft tools, if you purchase some of their additional solutions. However, we decided that it made more sense for us to go with an independent CASB that we could leverage for other things as well.
Since the initial deployment, we have looked to expand well beyond the initial use case to protect additional cloud-based environments as well as implement additional functionality, such as DLP.