I often use data collected by the Oracle Automatic Workload Repository (AWR) to help to diagnose performance problems. However, I often work on performance problems with application teams, rather than the DBAs. It is surprising how often that I don't have access to Oracle Enterprise Manager.
Somebody might say that the system was slow at a particular time. I want to get an overview of the database at that time, and I might also want to compare it to another time when the system wasn't slow. Later I may generate AWR reports using particular pairs of snapshots, but I need something to direct me to when an issue occurred, and hence which snapshots to compare.