Yes, your first question asked for use cases, a “what” question. The “why” question is the justification for the use case. (The “how” question is answered in the design docs.)
There are several reasons “Why”:
- We have a partial user-defined UI implementation. Two major UI customizations are available within Discourse - Default Homepage and Left Nav/Header Dropdown. Discourse has implemented the option for the user to choose the homepage landing UI and override the global default. To provide the user with complete UI customization, the user should be able to select their preference for the Left Nav/Header drop-down.
- I have users asking for it. The users have experienced the header drop-down menu on other Discourse environments. The feature is what they are used to using in a Discourse environment. This diverse option creates usability and unfriendliness issues between the various discourse environments that have implemented the header drop-down option as global. The last thing any organization wants is usability friction. This feature allows users to switch to what they are comfortable with in all their Discourse interactions - regardless of the website.
Those are the two top-of-mind justifications. I am sure there are others.