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

• What do they need?
• What do they care about most?

The primary goals of creating a building model are to enable collaboration early on a project and develop a true visualization of your project. The key stakeholders are the owner and the construction developer. First, the owner wants to see their interests reflected on the project and the developer needs details to build the project.

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

• What is the advantage to manufacturers for providing these families? (it's not free to create and provide them...)
• What is the benefit to you as the designer of using these manufacturer-provided families? Is there an advantage to using them versus the families provided in the Revit library?

There are several advantages such as increase their commercial scope, by providing their products to a wide sphere of professionals. As a result, it would be easier to show their products and guarantee their use by taking advantage of the early design of a project. It is advantageous for the designers also, it allows them to show the result of their ideas and help to sell their projects to the final client.

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?

If you get families directly from the manufacturer you got certainty on the model, and reliance to your design. On revitcity.com the models are more generic even that some show commercial families.