Entirely too much fun
Who would want to hack a bathroom scale? Well…
Let's say that some obnoxious show-off bloke brags to you about his expensive IoT bathroom scale. So you hack into it, causing it to increment his actual weight by 1-2pounds every 3-4 days. After a couple of weeks or so, he would become concerned about his “weight gain” and cut back on his food consumption – and it wouldn't do any good!
Then you reverse the hack, decrementing his weight. At first he would be gratified that his diet was finally starting to work. However, when you reach the point that the scale gives an accurate weight display, you continue decrementing, taking him 15-20 pounds “underweight,” and increasing his food intake wouldn't do any good!
It would never occur to the bloke that his expensive, state-of-the-art toy was malfunctioning.
(They're really not supposed to allow Internet access from this ward.)