[mattbrewsbytes]

Specifically to allow easier changes: abstraction, encapsulation and separation of concerns.

An example would be if you have a module that calls a REST API to get/put something (say time sheets for your invoicing app), then have that be its own module that is testable.

Create internal TimeSheet data structures that you pass to/from that module. The core functionality of your app should be implemented using the TimeSheet data structures and you can have tests that use those and then separate tests around calling an API.

New customer comes along and says they want to send you CSV files via SFTP (yuck, but they got money). You just have to write a new interface that works with exchanging those files and gets them into your TimeSheet data structures, the core of your app should remain unchanged.