I should have added. You'd create a 'model' to make it easier for end users to create questions in a more protected environment without having to worry about the complexities of your database.
So far, I've not had any reason to use them as I'm working with either 'nice' data or the users aren't allowed any where near.
That's likely to change shortly as in the next release, we'll be getting write-back and that needs models to work.
Yes, I saw the question. Hopefully somebody else will know the answer. I've never needed to use segments. I like to do data prep in the database and use Metabase as the presentation layer. That way, I can reuse stuff in other tools (Crystal Reports, PowerBI & Qlik mostly).