These trackers can be useful for many people. I was a bit abrupt (sorry, there's a lot going on right now) but the important thing is not to get hung up on the numbers. For one, the calories in those apps aren't always very accurate, and for two, the quality of the food you eat matters far more than the calorie count. So it's better to eat 200 calories worth of fruit or vegetables than 200 calories of chips or chocolate.
If done right, the number of calories in a food is determined by burning it in a bomb calorimeter. Our bodies are not calorimeters, so we aren't going to derive the same amount of energy from a food as is released when it is burned in a bomb calorimeter. So that's just one of the many reasons I don't like focusing on calories.
Food diaries (whether on paper or in an app) can be very handy for some people, to help track their intake. But the actual calorie count is pretty inaccurate and not terribly useful (as the only way to really know how many calories you actually need is quite involved - the common equations used can also be very inaccurate). We do know that the actual act of writing down what you eat (again, either on paper or in an app) can influence food intake, so that aspect of the trackers can be useful.
