The capability to easily call forth a needed log is what gives OnyX its unique value to me when I use it. Of course, I use the logs a lot. Not everyone will. Others will just want to use it as a way to view command manuals. It's got it all there.
OnyX's aggregation of disparate tools can make it useful in the same way a Leatherman multitool is useful. If you've never run a maintenance script, you can use OnyX to do so. If you run daily maintenance scripts, you can use OnyX to automate those scripts. There's something there for everyone. You just have to seek it out.