A better architecture 

Writing unit tests tends to push your code towards a decoupled design. Tightly coupled code quickly becomes burdensome to test, and so, to make one's life easier, a Test-Driven Developer will begin to decouple the code. Decoupled code is easier to swap in and out, which means that, instead of modifying a tangled knot of production code, often all that a developer needs to do to make the necessary changes is swap out a subcomponent with a new module of code.