I agree with the others that for most cases, the timer is a waste of money.
But if you are gone from home for long periods, say for over a day or two, the timer can save you money. Not due to the timer function, but because you can use it as a switch to turn off your water heater so it is not always trying to hold your water at the temp setpoint when you have no chance of using it.
For a much larger money saving thing to do, check the thermostat setting and set it for the lowest temperature that provides you with the amount of hot water for your needs.
My first apartment was total electric, and it took me a couple of months to notice the HWH timer that was installed in every unit. My wife and I began to use the timer, then realized that the water was very hot to begin with since we were concerned with the water temp due to our newborn daughter. I can tell you that from reducing the water temp we dropped over $100 per month on our electric bill!
The prior tenant really had it cranked up...