Build flags in Go are a versatile tool that allow developers to manage the features and functionality of their applications efficiently. They enable selective compilation of code, permitting the inclusion or exclusion of specific features based on build tags. By utilizing this mechanism, developers can easily experiment with new functionalities, such as adding various greeting options in different languages, without deploying them in production builds. Moreover, build flags can help streamline testing by controlling which tests are executed, particularly beneficial for speeding up workflows when dealing with extensive end-to-end tests. This lesson highlights effective strategies for employing build flags alongside the init function to dynamically configure command registration in CLI applications, offering a powerful means to tailor application capabilities based on user requirements or testing scenarios.