Decoupling code from specific implementations is a crucial practice in software development that enhances flexibility and testing capabilities. By transitioning the `count words in file` function from relying on a concrete `os.file` type to the more versatile `io.reader` interface, developers can now handle various input types, including strings and byte slices, with greater ease. This approach not only simplifies testing, as it avoids the complexities of file handling during tests, but it also enhances the function's applicability to other data sources, like TCP sockets or standard input. Emphasizing the importance of interfaces, this strategy demonstrates how decoupling can lead to improved software quality, setting the groundwork for future enhancements like supporting command line arguments and file inputs.