The longer you've been working as a DBA, the more you've encountered things like these:
The time on a table changed and you don't remember doing it. You poke around but can't figure out who changed it.
There's someone you don't trust that you have to give access to. They are the kind of person that will look at things they are not supposed to, but you have to give them access.
There's someone who always messes things up when they get access. You know they'll do it again and you'll have to undo the damage. Would be nice to know what they are doing.
There's a contractor or a new employee that need access. You don't know them yet and not sure how reliable they are. It would be nice to be able to keep an eye on them.
Too many people have too much access and you're losing control. You try to figure out how to reduce the access they have or revoke some privileges, but this is pretty much what everyone claims they need to do their jobs.
You've been through an audit and the auditor wanted to know all sorts of things about the activity. Questions you just don't have the information needed to answer.
You probably spent many days chasing down the answers to these questions or try to find ways to collect it. The security in the database is set properly. The problem is that you need to know what people are doing with the access they have. This knowledge will give you more control.