You don't even have to think that far ahead - consider some code you wrote in week 1, that was then modified by a colleague in week 2, now you return to the code in week 3 - what does the code do? Any mental model you had at week 1 is now irrelevant as the code has changed in unknown ways. What is the current intent of the code? Do you feel safe modifying it, despite the fact you no longer know how to manually test all the end points? You only have to miss one to have clients and managers breathing down your neck when the blame game is played as to who broke the app in production.