The key to building reliable applications is a combination of testing and monitoring. Testing ensures that each pull request works before it's merged and deployed to production, while monitoring detects issues that may arise in production, such as database rollovers or third-party outages. By separating testing and monitoring, developers can catch issues early on and avoid surprises for their on-call engineers. The Checkly CLI allows developers to reuse synthetic monitoring checks as an extra layer of tests, providing an additional layer of protection against application failures. This approach helps identify issues in both the application code and the monitoring setup, enabling a more comprehensive testing and monitoring strategy.