It's actually pretty simple; any time your phone is lost or stolen, you're going to want to revoke its access.
Logging into your account from a new device won't work if it's not a phone you've already set up. The same is true if you have to get a new phone.
The authenticator is a two-step authentication application developed by Google and released as part of the Google Toolbar on 29 July 2009.
It provides a six-digit, one-time use, number as a way to prove identity when logging in to a site or service that supports two-factor authentication.
The Authenticator app is a small app that generates a unique, time-based, 6-digit number that you can use to log in to your Google Account.
When you try to sign in to your account, Authenticator generates a verification code, which you will then need to use to complete the sign-in process.
To use Authenticator, you need to have a mobile phone that can read text messages. If you don't have a phone, you can still use Authenticator to generate verification codes for your account.
This can be useful, for example, if you don't have a phone and are traveling, or if you have a phone but don't have access to it for some reason.
