What are the primary goals of creating a building model? Who are the key stakeholders?
I think the primary goal is to bring a space to life. It's crucial to be able to see the 3D version of a space - for both the designer and the client. Architects can make informed adjustments to their plans, clients can specify/observe what they want, people in construction can see the building more clearly, etc.
How much detail should you include in your building model? How do you decide?
I think the building model is for the architect and clients to get a feel for the space. I feel the flow and the form is the most important, but the more detail, the easier it is to imagine. In this case I feel detail as in building materials, finishes, adding furniture can really transform the model. Other details as in annotating dimensions on a plan is also helpful, but less conducive to imagination.
Many door and window manufacturers provide Revit families for doors and windows that you can specify for your building design.
I think a key advantage could be having designers build their models to the precise dimensions of doors + windows from certain manufacturers. Once the designers are happy with their model, they may be more likely to want to keep those building elements.