Exchanging information between the browser and the 2FAS app on your phone is similar to speed dialing. If we want to call John, we don’t enter his phone number because remembering it doesn’t make sense. When we met him for the first time, we entered his mobile number and assigned him the name John in the phone book. Later it is enough to choose a contact named John and call.
It is similar when the browser asks for the code for Twitter.com, for example. It sends a request to the 2FAS app and the app is asking you to select a code/token, because the app doesn’t know which token should be assigned to the Twitter.com domain. That’s why the first time, we need to bind the domain with the token. The next time you log in, the browser will ask for a code for Twitter.com and get an answer with the correct token/code immediately.