Behavior change research shows that putting money on the line makes it much more likely that you'll stick to your habits. It's not being used much though because most people don't want to put money on the line haha
We think part of the reason is that your money is traditionally going to the startups (eg Beeminder, Forefeit). We solve it by donating it to effective charities.
I built it as a pretty minimal PWA to get around app stores and allow browser use. Apple makes it a bit hard to install (open in safari > share > save) but I wouldn't have done it if I would've had to go through all the app stores.