ngrok's Request Inspector and Replay functionality offer developers a convenient way to log, inspect, and replay traffic through their tunnels, saving time and effort typically spent on testing code logic. By launching ngrok and accessing the local URL, developers can view all requests made through their tunnel, validate data sent to and retrieved by their app, and verify webhook responses. The Request Inspector provides detailed information about each request and response, while the Replay functionality allows developers to replay requests with or without modifications, making it easier to test and debug their code.