I honestly can't think of a reason I'd get a smart plug as I don't think I have any electronics that would justify having one. I'm also not very much of a scheduler, as I don't have many daily duties I must perform, other than getting up, having a shower, breakfast, etc.
Makes perfect sense for maintaining aquariums though!
I know a few people who have small fridges in their basement or rec room for beer nights. They don't leave it plugged in. However, it does take a few hours to cool down beer. It's not always possible to know when you should plug it in on a Friday morning as you may make that decision a little later. With a smart plug, you could turn it on at work, or at least check to make sure it is turned it on by someone else.
The same goes for any other appliance. It's great for people who are always forgetting to turn on or off this and that because they can check with the phone.
It can even be a safety feature in the case of more dangerous appliances like space heaters. Or those that just waste electricity when no one is around like a fan or lights.
Those are some good examples of their usefulness. I didn't think of those kinds of things where they actually would be useful. Thanks for providing more insight! :)