Say you install a new electric resistance water heater, and the UEF is 0.92 (a value arbitrarily picked from one water heater I looked at). UEF is apparently based on a 125F storage temperature, with some draw profile(s) I don't know the details of, and 0.92 means if you put in 100 kWh of electrical energy, you get 92 kWh into delivered hot water.
So worst case, 8 kWh is standby heat loss at the tank (I'm not familiar with the standard, so some of that 8 kWh may be losses that don't vary with tank temperature). Say ambient temperature at the water heater is 70F. Then that's 8 kWh at a 55F temperature delta. If you are running your water heater at 140F, the temperature delta is now 70F. That would make the standby heat losses 70/55 * 8 = 10 kWh, worst case.
That means best case, if you turn your modern UEF 0.92 tank electric water heater down from 140F to 125F, and your usage pattern is at all close to the draw profiles used in the UEF rating standard, you'll be reducing the energy usage from 102% to 100%. I.e. you'll reduce your water heater energy usage by around 2%, best case.
Now if you have an older water heater with a lower UEF (if it were measured), the savings would be greater.
Cheers, Wayne