If we have tested our code and found that the functionality and everything else is fine, then why do we need monitoring?

The production load might be different from what we tested and found, there could be human errors while conducting the day-to-days operations, someone could have executed a wrong command or added a wrong configuration. There could also be hardware/network failures that could make your application unavailable. How long can you afford to keep the application down? Maybe for a few minutes or for a few hours, but what about the revenue loss, or what if it is a critical application for carrying out financial transactions? We need to respond to the failures as soon as possible, and this can be done only if we perform early detections and send out notifications.