
With our modern version control systems, we could assume we know for every single release we make the exact source code revision that corresponds. But, in the hurry of a bug found by your dear clients (or your product owner), you can't find the corresponding tag in your VCS, it will make the whole patch process really difficult. You will have to find the exact revision, extract it and debug to patch (or backport an existing patch).
I will explain a little trick to make the find the revision thing a lot easier (I am assuming you're using maven to build your projects) : maven-buildnumber-plugin