At first, I am asking what kind of Authenticator you are using?
Google Authenticator?
So, my question is under what circumstances is one asked for an Authenticator code?
basically, Authenticator is an additional security system that must be used in order to enable accessing or doing something that is related to security reasons. This is part of 2FA.
If it is not only about in COinbase, commonly Authenticator is used for:
1. Accessing or logging in to a certain platform, exchange, or site that requires authenticator code for additional security. It is very important to minimize being hacked or even abuse login access by someone who may know your email and username and password.
2. Authenticator code is also needed if you are willing to withdraw the assets from an exchange. It is also important to ensure that you are the real owner of the assets so that it will increase the security reason.
Further, please check these:
https://www.zdnet.com/article/better-than-the-best-password-how-to-use-2fa-to-improve-your-security/https://www.pocket-lint.com/apps/news/141559-what-is-two-factor-authentication-and-why-should-you-use-it