[frankdejonge]

I look more towards optionality myself. Take for example the encapsulation of randomness. Depending on an abstract notion of randomness (an interface) that decouples you from the implementation is both useful for testing as it is for maintenance of a system. For tests, removing randomness entirely makes it deterministic, allowing you to tests for exact matches instead of approximations. For systems, at a smaller scale get away with a reduced amount of randomness, while systems at scale require more sophisticated code for this. You don't want to replace all of that code in all instances, but rather leverage the capability and replace the implementation.

Another angle is the encapsulation of storage. Using in-memory storage for tests makes Ci pipelines very quick and production storage might evolve over time to accommodate scaling requirements (sharding and such).