I plugged my 2nd CPU fan into the AIO header b/c then I can set a separate fan control scheme. The CPU_OPT header shares same fan curve as CPU fan, which is probably just fine normally, but I'm re-using a 120mm fan that's too loud at full speed so want the separate control and I'm out of fan headers on my z390-E.
The only downside is that the AIO header is 100% at boot and stays that way until you get to Windows and AISuite takes over fan control.