User names are already displayed by default as long as you are logged in.
The option to whether or not display the username you are referring it's only for non-registered users, the system will not guess who is the person who asked the question by magic, that's why the login function exists, to store this kind of data and information for every user in the database so the system doesn't need to constantly ask - is this John? every time you access the site.
But if you are logged in, the system will know that it is John indeed the next time you access the site with a logged in account. That's why you have an optional field to either display your name/nickname to be asign for that question or to ask a question as an anonymous person, but if you have an account and are logged in, the system will know that it is "John" who's asking the question and will automatically assign the question to John.
If you want to avoid the need of always fill the "who's asking" field every time you come back to the site, just create an account.