This isn't necessarily true, especially with consumer GPUs. Some actually can clock higher with less voltage. It's pretty rare, and mostly comes from factory overclocked cards. For it to help you need a card that 1. Thermal throttles 2. can sustain its max OC with less voltage than set at the factory.
In that rare case you are removing thermal pressure which allows you to clock higher for longer.
It's the silicon lottery though, and (often) lazy board partners just smashing the voltage up as high as the chip maker allows. You definitely won't get more performance from a datacenter GPU this way.