Can anyone elaborate on why you'd define ViewData["MenuData"] on every action for something like a dynamic menu?

I have a simple partial view which renders a menu, I render this menu from within a master page. This is intutive for me comming from ASP.NET WebForms, but the only way for me to populate the menu is to pass ViewData["MenuData"], but then I have to do this in every controller action. It does feel a bit stupid, that I would have to define this view data every time.

In terms of testability and what's ASP.NET MVC-ish how should I approach this?