Not sure how it works on the Asus but on my MSI it is required as you found out for OC's that require more than 20A. At least that's what it says after it applies my 4,5 OC. And as you i only have an 8 pin plugged in and it's good to go.
Well I guess it depends on your CPU which you did not mention and the rest of the system, I'm running my i7 7820k fine with a 650w powersupply (same brand different model and does have 2 8pin for CPU)