Banks need to know who is accessing their data for compliance reasons. The other use case is related to creating, managing, and auditing databases. Some organizations cannot audit their databases because it involves lots of heavy processing, so they would rather have a third party do the database monitoring. That's where Imperva comes in. My enterprise clients use Imperva on-premises, but some customers are using FlexProtect, which is a cloud-based solution. About 10 security engineers at my company use Imperva currently.
Imperva's most valuable features are ease of use and log correlation. I also like the ability to trace activity from the host machine to the server access, the user details, and the exact query that they executed. If a database administrator or application process executes a query on the database, people know exactly what was executed, including all the variables.
For example, it will tell you how many rows they were trying to select on which table, and you get the OS information, network details, IPs, and the user. That fine-grained auditing available on the platform makes life easier and helps explain anything happening in your database.