When you first learn how to code, the goal is just to get something working. To get going, you can make a simple layout, add in some data, respond to some user input, and throw all that into one giant class. When building an app, you not only want to make it work now, but you want to make it work in the long run. When creating a project from ground up, you want to keep in mind that features are likely to get added, dropped, evolved, e.t.c.