With event sourcing you have a system to log any events you want. The main purpose is to track domain events, but you can also track login success and failures, who access a piece of information. The side effect of event sourcing is that it gives you an audit log. It's of course useful later on when you want to extract new informations from the events. Another good scenario is for application where you need to track who do what on the system for legal reasons, like for Hipaa compliance.