Logs are on the systems, why do I need this?
Because Digital Forensics & Incident Response is expensive -- likely the highest billable rate among most IT/security practices.
Why? Because it is a very skilled, but meticulous and time-consuming activity and my team has done our fair share of it. Most often, the bulk of the time is spent collecting often volatile evidence from countless systems in hopes that enough of the attacker activity is still traceable. The best evidence sources are often the ones least available at the time of the investigation -- logs. Why? Because they roll over, or get deleted, etc.
If I walked into an organization that had centralized logging, I could probably cut the IR effort in half because a huge amount of the data I need is there ready to be queried. This allows me to perform deeper forensic analysis only on systems that exhibited noteworthy activity.