What are the primary goals of creating a building model? Who are the key stakeholders?

The primary goals of creating a building model is to understand what your client wants and be able to create and clearly communicate your ideas and concepts to them. The key stakeholders are yourself, the clients, and the construction workers. First, the you have to be able to create a design based on the desires of the client, but then you have to ensure that the design is structurally sound and even possible. If the construction detail falls short in the design process then it's likely the design won't be approved, that there will be delays, or that a problem will show up later when it is more difficult to reassess.

How much detail should you include about the composition (layers, materials, thicknesses) of your wall, floor, and roof assemblies at different stages of your design process?

When you're first creating your conceptual design you shouldn't include so much detail about the composition of wall, roof, and floor. At that point, I think it's sufficient to take into account the material (face) of the foundation. For the preliminary design, I'd say that the same is true. It isn't until the design development when you really should take into account at least the thickness of the materials especially when considering floor height, roof height, and wall width. You will need to ensure that your designs can be accommodated by the foundation. It isn't until construction documentation that you really should do a deep dive about the composition of the materials to make sure everything will be compatible and properly functioning.

Many door and window manufacturers provide Revit families for doors and windows that you can specify for your building design.

The advantage to manufactures for providing Revit families is that it allows for designers to actually use them and present them to their clients. This creates the possibility that the clients really like that specific family and then it turns out that it actually exists. I think this is really just advertising for manufacturers who are trying to get more customers.

I think that maybe the benefit of using these manufacturer-provided families is that it allows us to use more intricate and creative designs without having to go through the trouble of creating them ourselves. The benefit of using these families versus the families in the Revit library is that there are just so many more options to choose from that might be specific to your needs or even inspire you to go in another direction or follow a different idea. It just gives you a larger pool to pick from.