Accumulate as many days in a row as you can. Amass a treasure. If you break the streak, you lose everything - and loss aversion is a powerful motivator.
You can also create a habit of not doing something, like "No alcohol". These are labeled as done for the day until you manually "undo" them the moment you do something you are trying not to.
Having to admit a failure to this app is a surprisingly powerful thing. You pause for a second and your conscious self can use that second to take control over that little devil inside of you.
Building new habits is hard. Anything that helps can be the difference between sticking to healthy food, meditation practice, language learning, no sugary drinks, or whatever you want to do (or not do)... or failing.
of course, there is more to successful habit-building than just streaks. I recommend the article Sticking to a Habit: The Definitive Guide by Leo Babauta.
Let me know about problems, feature requests... whatever you need.