ASP.NET MVC3，为什么dropdownlist甚至在强类型视图中依赖于viewbag

I'm new to MVC, so maybe this is a stupid question - I'm trying to get my head around strongly typed views in asp.net mvc. I'm working on version 3. If I have a project with 2 models - say Person and Department. A person must belong to a department. So I have my Department model (and I've generated my controller and CRUD interface):I'm new to MVC, so maybe this is a stupid quest