Dev mode is when developing by the programmers. Production means used by others.

When I am working with a service, I want to know immediately when there is a problem.

When I push my code to be used, the dev flag is off and the code does stuff as normal.

For example: if I am making a payment service using a third party, I use sandbox and fake cc numbers. I want things to fail loudly on my computer so I can make things fail silently. And safely, when it’s used with real money. But I can only do that if I know there are issues. And only logging lets some bugs be unnoticed , particularly if a cluttered log. So being loud in development saves tears in production