Creating and running an application in isolation mode makes your application run without any issues. However, when working on a real project, your application will be executing in an environment where many external interfaces may interact. In such scenarios, is your application capable of handling such communications? Can it handle all kinds of data from these external applications? There will be many users who will try to use your system; some may use it properly, and others may try to break your system. Can your application tolerate such interactions?
There may be problems with both types of user. Those who use your system properly may make mistakes by entering incorrect data or may forget to provide the requisite data. In the event your application has logic based on the user's date of birth and the user enters some text data, your...