The contents of the box are
1. A programable timer light switch: https://leviton.com/products/vpt24-1pz
2. A 120 volt controlled contactor. I'm not entirely sure which one it is, but I think it is https://ca.rs-online.com/product/eaton-cutler-hammer/c25bnb2...
The box has a 240 volt circuit and a 120 volt circuit into it. The 120 volt circuit is connected to the programable timer light switch, which is then connected to the control inputs of the contactor.
The 240 volt line connects to the contactor and then on the other side of the contactor it connects goes out of the box and to the water heater.
For me I've programmed my hot water heater to turn off from 4pm to 11pm on weekdays, which avoids heating the water during peak time of day rates at my location. (I typically shower sometime during that period.)
I can open the box and press the switch on the timer if I ever want to temporarily override the program.
That all said, I don't have any measurements to tell me if it is worth the expense. Although it is quite clear from my hourly power usage when the hot water tank switches on at 11 pm.