For the coffee maker, maybe being able to set a schedule to brew in the morning.
For a water heater, participating in a utility program where they modify your temperature sweeping in exchange for a reduced rate or similar incentive.
What vendor and model water heater did you get? Useful smart features are of the variety that the manufacturer would never enable off the shelf, such as monitoring magnesium anode deterioration so that it could notify a user when it is time to replace the anode. It's against the interests of the manufacturer because replacing the anode extends the life of the heater.
Lots of interesting suggestions/applications in response to my initial comment. My local electric utility has a smart grid, but offers me as a consumer none of the so-far-listed reasons to connect to WiFi for electricity savings (e.g. no time of use metering)... but it would be cool if the anode deterioration could be monitored [I'll check the manual].
There is an entire operating system and a massive amount of functionality in your home that can be unlocked when devices have features like that. It's one of most active projects on GitHub and there's a huge community that knows the value.
The only downside is companies trying to scoop up that data for their own purposes and when companies disable perfectly working products because they claim the servers are too expensive. The Home Assistant community makes a big point of recommending products that guard against issues like that.
Some people have solar installations, but do not have 1-to-1 net metering from their power company. For these people, having a connected hot water heater allows them to use their own solar power for heating water when they can, lowering their power bill.
Essentially any high-consumption electrical device can similarly benefit, especially ones that store energy such as hot water heaters and electric car chargers.
My new water heater came with WiFi, and I just cannot understand why my tank needs-do anything more than just heat water..?