The purpose of the app is to store your todo items, but also to demonstrate how your choice of the task order affects the tasks lateness. It is designed as a guide so you can help manage which customers you need to communicate with or to get a somewhat more realistic view of your overall position.
You would use this app when you want to know the status of your jobs. You should update the time remaining and due dates if needed and then look at the various strategies to see which strategy would be the best in order to meet your goals. Keep in mind that it is not a good idea to switch strategies too often due to additional time for the context switching that you or your team will go through.