You have to do some clever accounting for it to relevant in some circumstances. Some data centers are entirely powered by hydroelectric power, and use of that power does not cause any carbon emissions.
In this case, every kWh spared by your data center still is a potential reduction of carbon emissions : let your hydroelectric power plants run at full power so they still provide this kWh, thus it doesn't have to be produced by the carbon-emitting power plants. Leading to reduced carbon emissions.