Without a pattern like MVC, your code could turn out to be hard to maintain as it could have low cohesion and high coupling. Those are fancy words, so what do we mean? Cohesion is about focus and what the class should do. The lower the cohesion, the more different things are performed by a class and therefore it has no clear intention of what it should perform.
The following code shows what happens when a class has low cohesion; it does a lot more than storing data about an invoice, such as being able to log to a file or talk to a database:
Invoice details total date validate() print() log() saveToDatabase()
Now we have introduced new dedicated classes and moved methods out of the Invoice ...
