The web interface can be reached on http://localhost:4040.
In the web interface you can view both the request received and response sent. The default view of the request and response is a summary of the post data, but it can be viewed in raw format or binary format. This is really useful when debugging webhooks.
Another great feature of this web interface is the replay functionality. Instead of having to run through the code again to get to the problem, you can just hit the resend button and the request will be resent.