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

Communication is, I would argue, the primary goal of a building model. To describe your ideas clearly and cogently and unambiguously to the client, who is the stakeholder.

The stakeholders here are the biologists at the field station. They need a relatively inexpensive work station where they can observe their preserve, with the appropriate equipment, and discuss the data amongst themselves.

One should provide enough detail to communicate the relevant aspects of your design to the client, but no more. Starting with a simple model and building in complexity as you progress is a good way to go.

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?

Levels of detail required:

Least: conceptual design (this is more about questions of style, interpretation, etc.)

Most: construction documentation (construction workers need to know exactly what to do!)

What are the advantages of getting the Revit component families (for furniture, equipment, and fittings) directly from the manufacturer versus from an online sharing website like RevitCity.com?

Advantages of families include easy sharing across designers. Also, in contrast with online components, we are sure that the component we are using is indeed the one that will go into the construction. It benefits manufacturers, too, in the sense that their components can be more widely known by the design community.