Mostly used user id is in fact your email address.
Because of this there could be a default value to pick for the user name when you add a new password entry. One could then enter the mostly used email as the personal default or whatever user id one most commonly use.
Benefits:
- Speeds up entering new password entries into authpass.
- Ensures that you get the user id correctly entered for your new entries.
- Increased ease of use (usability).