Writing testable code is important so that code can be easily tested through automated unit tests. Testable code is written to comply with SOLID principles and is separated into isolated units (classes/objects) that each have a single responsibility. There are different types of automated tests like unit tests, functional tests, and behavioral tests. Automated tests have benefits like proving code works as expected, protecting against regressions, and allowing refactoring with confidence. Dependencies between code units should be managed carefully to keep code testable and decoupled. A dependency injection container can help manage object dependencies and simplify code.