Correlation is also why we started this. A little story: we were using Cacti on a cluster of ours. I was showing a friend of mine the graphs, only because I was explaining our infrastructure setup. I noticed one had unusually high CPU load. The server with the high load was one of our SQL database servers. I SSHd into it and sure enough it was MySQL causing the load. But ... why? So, after so log file browsing, we found that our site was being scraped by some bots, and the particular page being scraped was doing a sorted query on a small range of data, which was causing the entire table to be sorted then scanned, which was causing high CPU usage, we thought. So, the correlation there between high CPU usage and the cause was less direct, and a simple graph was only enough to alert me of the fact that there was a problem on that machine. Plus, I wasnt even alerted - I stumbled upon it.

So, this is something we're aiming to fix.