It's probably not a good idea for someone who's already diabetic, but it can be good for insulin resistance, and maybe prediabetes, depending how advanced it is. There are also different ways to do it, some more extreme than others. The moderate way to do it is 16/8, which means you don't eat for 16 hours of the day; there's only an 8-hour window in which you consume food and caloric drinks (let's say noon-8pm). Supposedly letting your stomach/intestines fully empty and digest is one of the biggest benefits, plus it can regulate insulin levels because insulin spikes every time you eat so if it doesn't spike at all for 16 full hours a day it can sort itself out.
Of course there are people who take it to extremes, like everything else in the world.
