Has anyone found a single open-source app that supports both mobile and desktop though? That was the attraction of Authy before they killed their desktop apps.
The main purpose is that people won't get phished as easily or if they reuse passwords it can't be abused. Or if password was to leak for any reason.
It makes it unlikely someone has access to both your password and the TOTP URI. So, if you leak your password on a public forum (for example), the person who gets that is not likely to also have your TOTP info.