The oAuth redirect URL will be the sever/page/script URL that process the Step 2/3 of regular user oAuth
The code that exchanges the ?code for an access and refresh token.
You’ll need to get a new token for your account every now and again (and the first time) if the key is invalidated for some reason. So having an easy way to give the bot new credentials is a good idea.
Sure localhost works but then that means the key exchange stuff lives on your computer and if you need to give the bot new keys whilst not at home, you are stuck with a non working bot!