It's always helpful to keep some sort of diary. It helps you see how much insulin you use, what you eat, how many carbs you have a day.
I don't use the app, because I'm on a pump and that keeps track of things as well.
I agree! Although I don't do it all the time, I have on occasion stepped up the logging of food, BG, insulin -- even using a spreadsheet with fancy graphs etc... This provides useful feedback that I find helps to keep me "on track".