It's not a placebo. Setting the voltage slider to max unlocks four additional voltage steps you cannot reach unless you have the voltage slider set to max.
e.g. For a 2080 ti the max allowed voltage with the voltage slider set to default = 1.068mv. With the voltage slider set to max you can go all the way to 1.093mv, so you gain steps 1.075mv, 1.081mv, 1.087mv & 1.093mv.
The voltage slider doesn't actually increase your voltage like the old school days, but you can increase your voltage by following my oc guide listed
here.